If you are from Los Angeles, go to China Town. They sell 'em really cheap. like 5 for $10. If you have a business Lic. and tax id, it should help you get a discount at wholesale. You just have to call around and compare prices. Even some craft stores sell plain tshirt for cheap.
You should be lucky enough to not have to worry about buying a lot of maternity clothes. At least not until your third trimester. There is this great invention called the belly band which can cover the waistband of your jeans and pants when you start needing to leave them unbuttoned as well as cover and bits of tummy that start to peek out from under your shirts. Target has a maternity line by Liz Lange that has some really cute pieces for a reasonable price. But take advantage of empire waist shirts and tunics, cute jersey stretch dresses and invest in a few key pieces of maternity clothing like black slacks, jeans, and a few camisoles. Old Navy has a lot of great basics for a reasonable price as well, and they carry all sizes.
But it really is best to try to find non maternity pieces that will work for the majority of your pregnancy so you don't have to spend outrageous amounts on maternity clothes.