Charity shops, amazon, ebay, supermarkets (although only some of their stuff). Peacocks sometimes have things for a similar price, especially if there's an offer on/ with a student discount. I think there's a shop called store 21 or something that sells stuff really cheaply. Although I've only seen one, so idunno how many there are in the country, there isn't one in my town. Having said that, there isn't a primark in my town either.
H&M's sale can be cheaper than Primark. Look on their website as the in store sale stuff is randomly arranged and takes ages to look through.
Also Forever 21 is amazing value - really nice stuff for great prices, and they have great sales too. Just after Christmas they were doing a 3 for 2 thing on all their sale stuff, which was already up to 50% off, so I got tons of stuff for hardly anything. They do "flash deals" every day as well, and have sale items most of the time. I got some amazing skinny jeans for £8 the other day (Primark's are £9) - gotta be my favourite shop ever
You can also get lucky in TKmaxx - the have a section called modbox or something which sells their nicest (ie most fashionable ) clothes so its worth a look if there's one nearby.
I find that supermarket clothes really aren't that cheap, and are really bad quality. Same for Peacocks - most of their stuff is horrible.
Internacionale sells some really nice stuff, and their sales are so good, but they don't do online and their non-sale stuff can be really expensive.
But yeah, I'd recommend just checking sale items of well-known shops (even Topshop and River Island can have some great deals), especially online as it's easier to find what you want, and they're more likely to have your size.