Shimano used to do them to allow fitting IS Calliper to post mount for 180 mm disc.
The only other source is KCNC adapter
. <-- linky
It comes in 3 flavours for: 160, 180 and 203 mm discs. Front IS calliper have shorter mounts than rear IS calliper which in theory means that you can run a 160 mm disc on a post mount fork using the front calliper.
but, and it is a big BUT
there may be insufficent clearance so you may need to go to 180 mm disc - either fit rear calliper on front using 160 adapter or use front calliper with 180 adapter.
As an example, Shimano XTR m975 front IS calliper cannot run a 160 mm disc on a Fox post mount fork.