I've also had a lot of success finding fitness clothes on Zulily - right now they're having an athletics sale and I am down with these picks:
Another new favorite discovery: Fabletics (you know JustFab? Same people just with fitness clothes- great return policy, great prices, and you can order entire outfits. I bought a bunch from clearance a few months ago and love love love the pieces):
Where do you find reasonably priced workout wear?