I've used the general transfermations brand they sell at craft stores and Wal-Mart, and it works pretty damn well. You just have to make sure you hold the iron down on the transfer/shirt well to make sure it sticks properly.
You can probably try an online place like customink, cafepress, etc. for custom printing, but they only print on certain parts of the shirt.

Pic related, the things I transferred to the cosplays. (Center girl and the one to the right of her, Mio and Yui.)

Anywho, good luck, Anon.

>>5026517
AVERY and HP brand iron-on transfer sheets have been good in my experience, too.

Thank you so much, PantsuNugeruMon.