Apparently Gap's maternity yoga pants are fabulous (a couple friends swear by them, though I haven't purchased anything "maternity" yet other than a pair of jeans I haven't worn yet).
I actually just went on the REI outlet page and bought a some stuff on super-clearance including a pair of REI brand low rider yoga pants, and couple of shirts/tops [Sporthill, and Moving Comfort, I think] in 1-2 sizes larger than I normally wear. I figure there will be enough room at the bottom of the tops to fit through March-April ok. And after that, I'm sure I won't be running any more anyway! Between some discount coupons they had and the super-sale, I spent less than $50 on what would otherwise be very expensive (the colors are tolerable). I also would consider checking out Target's fitness clothes in 1-2 sizes larger.