boohoo.com is rather cheap and asos can be cheap, though there's also some expensive brands on there. Also most high street stores have websites. I love peacocks xD
Believe it or not - ebay! You can get some GREAT buys which are brand new rather than second hand, and just recently I ordered a few items all the way from Hong Kong for around £8 each
H & M do amazing deals online and Ebay is always another winner, although Ebay is trial and error...you need to know what your looking for. You can also pick up some amazing stuff in the markets these days.
I think 2nd hand shops are doing great just look at websites like ebay & nextonly.co.uk they both have wide variety of stuff sold by other customers who have used them.They are damn cheap.
Boohoo is cheap but everything can be ugly in it's own way on that website. H and M has a good online site which cheap stuff in the sale and I love H and M so is perfect for me.