A good place to find cheap plain t-shirts (I've found that the ones at craft stores are made weird, don't fit me well) is Wal-Mart. Check the little boys section. They have a ton of different colored plain t-shirts for $3.50 and if you need a regular adult size, they have those in the men's section for like $4-$5. They should be near the underwear and socks and all that stuff. But they are never consistently there, so it's a hit or miss kinda thing. Another good idea is a sports/shoe store. Maybe a bit more expensive but in addition to regular t-shirts they have plain baseball jersey type shirts--very cute with a stencil on them.
Yeah, you can tell I've done a lot of searching for shirts