Conde has them.
Sublimation, CrystalMPrints, Transfer Paper, Heat Presses, ChromaBlast Ink - Order Online! - DyeTrans.com
They are poly duck and print beautifully. Take a 14-15" insert.
One problem is the envelope closing on the back. Even with the use of a teflon pillow there is still a light area where the transfer crosses the seams.
Have planned on making a gradient pattern on the back to make it less noticable.
Or if you don't mind a white back... they are perfect.
The fabric is also available if you know someone who can make up simple pillow covers AFTER you imprint.
LEO