What I did was take off the waistband of jeans I already owned (try it out on a pair you don't like too much first!), sewed up the fly, and sewed a thick elastic (around 4") band around the top. Just make sure the jeans aren't too low-rise first, or you will definitely get "plumber butt" around the middle of the 2nd trimester. It's better if the back is slightly higher than the front. This works even better with skirts, because you can pull them up/down as you get bigger.
Similarly, any elastic-band skirt in a size or two larger than you would usually wear works great! Often these are really unflattering when you're NOT pregnant, but look fine when you are!
Another thing I did to extend the amount of time I could wear my regular shirts is buy mens small/large women's tank tops for underneath. You get kind of a layered look, but avoid showing the panel on the maternity pants. This stops working so well in the 3rd trimester, but is great when regular maternity tops are too big.
Old Navy has really good sales, but go in person because the sizes are different for each style, and some of the materials are really flimsy.