I'm the same, its really only office clothes I needed to get some alternatives for. I bought a few pairs of pants (3/4 and full length) that roll down at the waist from target in basic colours. They're fantastic. My problem was buying up a size in pants with a fly etc meant that they were huge on me everywhere else as I haven't put weight on through bum/thighs/ hips etc. They will get me through the whole pregnancy. I had a lot of longer style tops with room in them anyway so most things will fit all the way through.
You don't need to spend a fortune to get a few basics that will get you through. LIke some of the other girls have said, the chain stores have very reasonably priced maternity stuff which is still good quality.
Good luck