There is an amazing source called SupaColor at
www.Supacolor.com and the FULL COLOR prints they produce are amazing,
I found them after watching a couple of Tshirt guys on YouTube...
There is also a great source on the west coast for spot color screen printed transfers that take all of 5 seconds on a heat press... and are very affordable
(PS SupaColor DOES NOT ALLOW GANG RUNS, aka sheets with multiple jobs on them,... so be aware!
also there is
www.fmexpressions.com for single color screen printed heat transfers under 20 cents each...
anyway, Hope this helps!