I know you couldn't possibly fathom this right now, with the heat and all, but as you get bigger and winter approaches, you can live in sweatpants with draw strings and they very inexpensive. I bought mens XXX large with a tie string with the first daughter, as she was born in January. This helped save a lot of money. If you have to deal with work clothes, you may have to buy a couple of pair of pants at Motherhood Plus, but you can mix and match with a couple of tops.
I like Motherhood Maternity. Yes, they are a bit pricey but perhaps you could find a Lot of clothes on Ebay or Craigslist. Especially since you will not be wearing these clothes for long its a good way to save money. I've bought a few things and have been lucky enough to have friends that are letting me borrow their maternity stuff.
http://www.motherhood.com/plushome.asp
Old Navy has a new maternity plus size section, you might wanna check that out. i took a peek and it looks pretty cute to me! there isnt a whole ton of stuff in the section online but its better than a moo moo!!! Goodluck, i know maternity shopping can be quite frustrating!