I just got a bunch from Michael's last week. They were 2/5$. I have also gotten them at WalMart. Pretty much any solid colored shirt I can get for it. A lot of times WalMart has shirts on clearance and I just use those.
I generally order from www.jiffyshirts.com but I also watch Target and Walmart to see if i can find cheap shirts there too and I usually can find V-necks I like for me.