I use Avery and get them at walmart for about $9.87, its a pk of 5 transfer sheets. Staples has the exact same thing for almost $15 but they will price match!
I used www.printonit.com this year and was EXTREMELY happy with them. We have reworn a lot of our shirts probably a dozen times or more and there has not been a problem. I think these are the best transfers I have ever used.
I bought mine from hp.com - buy 2 get 1 free promo w/overnight shipping. Loved the HP brand (more than the Avery brand I ran out to Staples to buy when I ran out of HP's).
AC Moore has a sale right now for Dark transfers for $5.50 for a pack of 5. I bought some to give them a try but I haven't actually started my printing yet.