I only ever buy things with spandex in them from H&M. Because their sizing is so unpredictable. Also, I mean, the t-shirts with spandex in them cost like $5, and everything else costs considerably more, and once you've spent $5 on a t-shirt it feels like a crime to pay $30 for a pair of trousers that aren't even stretchy at all. So I don't.
(Although I should say, now that the Original Filene's Basement is closed, I have no idea where I'll get my really really cheap nice clothes! Used clothing stores have only so much, you know?)