I agree with everyone who says target... The Targets that I've been in have had a massive selection of fitness clothes (and fitness stuff in general). And they're usually really cute. Some of it you can buy off the internet if you don't have one near you, but there might be the issue of different brands/sizes/fits, you know... when a certain size in one brand fits you but the same size in a different one doesn't.
It helps to be able to try things on.
Walmart has some cute stuff and they usually have a bigger selection of bigger sizes, but most of the time when I go shelves are picked clean.
Honestly... the way I get past the super short, super tight problem is by sneaking over to the men's section.
I usually buys my shirts, sweatshirts and sometimes shorts over there... and then buy the pants from boy's section (I'm short). I know it sounds weird but they have alot of stuff that is exactly like the women's but not as tight. They might not have the bright pinks and lime greens, but I found the same black, blue, white, grey, red, green, etc. warm up suits they had in women's in the men's section but baggier and more comfortable to actually move around in.