Found A.S.Solutions Flat Mount Adapters for Post Mount and IS frames and forks Bikerumor

Flat Mount Adapter (160PM-160FM)This adapter allows you to use a flat mount caliper with a 160 mm. brake rotor on a frame or fork that was designed using the 160 mm. post mount standard. Compatibility: This adapter only works on frames or forks that have 160 mm post mounts. It will only works with 160mm. rotors.
Adapter for Flat Mount Caliper on 160 mm Post Mount A.S. Solutions

Now, that's not to say it can't be done. A.S. Solutions offers a few adapters that allow you to fit a flat mount caliper on a post or IS frame. There are limitations though. First, you have to have the clearance on your frame. Second, the IS adapter only works with 180 mm rotors in the front and 160 mm in the back.
Shimano SMMAR160P/D PostMount to FlatMount Adapter for Rear 160mm Rotor Disc Adaptors Bike

In back, flat-mount compatible frames use bolts going through a frame member, typically the chainstay, to thread directly into the caliper if a 140mm rotor is used, or an adapter for 160mm. Flat mount in an of itself is a design that did a good job of minimizing the number of adapters that may be needed, with the flippable front adapter that's.
Adapter for Flat Mount Caliper on 140 mm Post Mount A.S. Solutions

3. AS Solutions in Canada now sell an adapter for IS directly to flat mount, with caveats around what size rotors can be used and whether your frame has clearance. From their site: Rear IS mount (Frame): This adapter will only work with 160 mm. rotors. Front IS mount (Fork): This adapter will only work with 180 mm. rotors.
Adaptor Post Mount To Flat Mount ubicaciondepersonas.cdmx.gob.mx

Instead of ponying up for shiny new flat-mount calipers, get some use out of those post-mount ones with TRP's Flat Mount Disc Brake Adapter. With options for running 160mm or 140mm rotors, and added options for flat-mount to flat-mount pairings, these adapters will have you up and running and out exploring new roads in no time at all.
Adapter for Flat Mount Caliper on 140 mm Post Mount A.S. Solutions

Vermont-based Analog Cycles developed the Discord 180mm Flat Mount Disc Brake Fork Adapter to bring the braking on their drop-bar all-rounder up to par with its singletrack prowess. The two-piece design is made in the US, comes with stainless hardware, and is available in black or silver anodizing. Analog Cycles advises to check with your.
Wolf Tooth Post to Flat Mount adapter puts new brakes on old frames, forks Electric Vehicles

Front Flat Mount Disc Adapters $ 32.00 - $ 44.00 Select options; Post Mount/I.S. Klamper $ 244.50 - $ 317.00 Select options; I.S. & Post Mount Disc Adapters. IS or Post Mount, we've got you covered for rotor sizes 140 through 203mm. Mount screws with thread lock already applied come with all adapters. Machined to perfect tolerances here.
SHIMANO Adapter Flat Mount 160 mm 140 mm front, 14,50

A.S.Solutions offers three different adapters to run flat mount calipers on post mount frames or forks. These allow for a 160mm rotor on a 160mm mount frame/fork, and a 140mm or 160mm rotor on a 140mm post mount frame or fork. There are three different adapters depending on the size rotor and mount you're working with, and each has provided.
Found A.S.Solutions Flat Mount Adapters for Post Mount and IS frames and forks Bikerumor

Flat Mount (12) I.S. (11) Post Mount (14) Price. to GO. $5.00-$10.00 (3) $10.. These holes are perpendicular to the wheel, are 51mm apart, and require a special IS disc brake adapter. Flat mount disc brakes sit flush against the fork and on the rear of the frame. This standard of brake mounting is similar to post-mount disc brakes but use.
Flat Mount Adapter PARTS AND SPARES ACCESSORIES Whyte Bikes

Hope this helps! :) 00.5318.009.000 IS BRACKET - 0 IS (FRONT 160/REAR 140) INCLUDES STAINLESS BRACKET MOUNTING BOLTS. 00.5318.009.001 IS BRACKET - 20 IS (FRONT 180/REAR 160) INCLUDES STAINLESS BRACKET MOUNTING BOLTS. 00.5318.009.002 IS BRACKET - 30 IS (REAR 170) INCLUDES STAINLESS BRACKET MOUNTING BOLTS.
Adapter for Flat Mount Caliper on 140 mm Post Mount A.S. Solutions

However, flat mount brakes ffs. There is an adapter available from Canada but it works out about £30 after shipping. Anyone 3d printing then here? Flat Mount Adapter (IS-FM)
Adapter for Flat Mount Caliper on 140 mm Post Mount A.S. Solutions

IS mount to post mount designs do not suffer from this issue since the bolts screw in from the top, and pinch the caliper between the bolt head and the adapter it threads into. Flat mount screws in through the bottom, and pinch the frame between the bolt head and the caliper it threads into. 2. Reply. 1 / 3.
Direct mount to flat mount adapter Kinning Cycles YouTube

This adapter allows you to use a flat mount caliper on a frame or fork that was designed using the IS mount standard. Compatibility: Rear IS mount (Frame): This adapter will only work with 160 mm. rotors. Front IS mount (Fork): This adapter will only work with 180 mm. rotors. Tested with Shimano, SRAM and Campagnolo flat mount calipers.
Adapter Flat Mount 140mm przód

The Wolf Tooth Post to Flat Mount Brake Adapter allows flat mount brakes to be added to a frame or fork that was designed for post mount brakes. This is a vital component for cyclists who want to run a road groupset on a mountain bike frame. It is important to note that this is a +20mm adapter, which means a 140mm post mount native size will.
Post to Flat Mount Brake Adapter Wolf Tooth Components

Please use the IS to Flat Mount Adapter Drawing (Letter, A4) to help determine if it will work for your application. IS to Flat Mount Adapter Drawing: Letter paper size: Letter Drawing; A4 paper size: A4 Drawing; When printed actual size the drawing can be used to check the compatibility of your frame or fork as shown below.
Shimano Road Disc Brake Adapter (Black) (Flat Mount) (140/160mm Front) Performance Bicycle

The adapter sits below the line of the post mount. This minimum required clearance may present issues on a small number of frame/fork designs. On paper, Wolf Tooth's design doesn't appear to add anything substantially new to the pre-existing and relatively unknown flat mount adapters from Canadian-based A.S.Solutions. And while Wolf Tooth.