My sister made shirts for us for our last trip.  She got both the shirts & iron-on transfers at either Michael's or AC Moore.  I've seen the transfers at Wal-Mart, though.
She used a group picture of Mickey, Minnie, Donald, Goofy & Pluto - but the site she got it from is no longer functional.  Are you looking for URLs for the fonts & clips?  Under the picture, she wrote 
"xxxx Family Vacation
Animal Kingdom Lodge
May 19 - 29, 2001"
We got quite a few comments on them - especially at AKL!
My suggestion would be to get the transfers first so you can read the directions about the shirt's fabric content.  They're specific about the cotton/poly ratio.  You need to wash the shirts before transferring - but don't use fabric softener.
Make sure you trim the transfer instead of using the entire 8.5x11 sheet - otherwise you end up with a really stiff shirt front! 

  It also seemed to peel easier when using the whole sheet.  Make sure you really get the edges good when ironing - or they'll crack & peel.
I'd also suggest that you wash the shirts again after you're finished - this time using fabric softener.  I always wash mine inside out.
The other thing I'd do differently is to make more than one set of shirts.  We wore them our first day & then wished we could wear them again.  We ended up washing just so we could wear them again! 
Good luck,
Deb