First pregnancy, I used larger sizes of knit stretchy type pants from WalMart, went from a size 10/12 up through a 16/18 at the end, and also some real maternity pants, including the jeans. I got one pair of the Secret Fit Belly form Motherhood at the end of that pregnancy and fell in love witht them, and wanted all my pants to be like that!
http://www.motherhood.com/maternity/secret-fit-belly-jean...
The top is like a stretchy tube, so in the beginning, you can fold/roll it down, so they only come under the belly, or halfway up, and then when the belly gets bigger, you can pull it all the way up and over the belly. Unfortunately, they are very expensive....so I took all my older maternity pants with just the front panel, or the under belly style, and went on eBay to this store:
http://stores.ebay.com/A-Great-Expectation_maternity-nurs...
I bought enough belly bands in coordinating colors to go with each of the pants and had a friend who could sew, cut off the old waistband (using my one pair of Secret Fit Form Motherhood as an example) and sew on the belly bands, and they were the BEST - you could even do it with non-maternity pants, if you find some on sale that fit your thighs and but, but won't zip or button, you just cut off the waistband starting right uner the beltloops at the back, and curving down to cut off most of the front, down to the bottom of the zipper,a nd then sew on a tube/band! Good Luck!
You should try on, and start with just a few pairs, a kahki, a black and a jean, and then try mroe as your belly grows, but I presonally think the bakd/secret fit belly is best cause it takes you all the way through all the different sizes and will still work a while after in that funny postpartum time when your belly is still not back to flat yet.
Jessie