Where is the best place to buy work out clothes for cheap?

I am a 190lb 18 year old looking to lose weight. Where is the best place to buy stuff for that? I have some tee-shirts here, but would rather have tanktops. I sweat a lot working out as it is usual, but am wondering if there is anything special that would not make it look so wet and be uncomfortable for shirts? I have stretch marks everywhere, but oh well...I still want some comfy shorts or capri length things to work out in. Also want some sweatbands. Is ebay the best place?