2 questions in 1!

I'm going to ask my BMs with 'bridesmaid' tshirts! I want to use the tshirt transfers.

1) How do the transfers work? Is it just 1 color writing? I'm really clueless w/ crafty stuff.

1) Where can I buy plain tshirts? I'd love to find navy, or baby blue. I need a XXL (or XL, if it's preshrunk), a L, and an XS.

TIA!!!!
(and Sorry for all the Qs this morning!)