american apparel sells plain & ringer dog tees in a lot of colors. daisy has several of them.
they even offer to sell them wholesale.
Baby Rib Dog T
Stylishly fitted for small to medium-sized dogs.
100% Baby Rib cotton, combed for softness and comfort
Cut to avoid doggie mess
Contrasting collar, cuffs, and hem
Durable double-stitched bottom hem and sleeves
Superior screenprinting results
http://www.americanapparelstore.com/4905.html