What I'd like to say:
What is it to them what you do at the store? Hell, enough guys buy make-up, lingerie etc. for their female partners don't they? How do they know you're not doing the same? It's not even their business. Just chill and go in like you don't care.
The reality:
People assume, especially if you're nervous and/or try to hide who you are. It's easy to say "relax, you'll be fine", but I know it doesn't feel that way. My heart was racing the first time I went into a charity shop to browse - I just skimmed over the men's feeling like it'd look too odd if I browsed further, even though I've bought past boyfriends clothing as presents. Just something about knowing why you're going in there and thinking everyone else can tell. Having said that though, I went into TKMaxx, made for the men's part after noticing other women go down there (sounds daft, maybe), and was soon browsing the Tshirts with a growing smile, even though I still felt a little twitchy - it also felt great

Still, Ebay is your friend - that's my main place to shop these days, just 'cause it can be a lot more affordable

Not to mention, especially for someone with alternative taste like me, you're more likely to find things you can't find on the high street.