I usually get my stationery from the back of TK Maxx but I have heard from a few people now that not all of their shops actually sell anything other than clothes. So if you live in the UK it's worth checking out the back of TK Maxx and kinda rummaging around in between the books and notepads but I can't guarantee you will be successful, sorry!
The other place I get stationery from is eBay. This is the shop I usually buy things from: http://stores.ebay.co.uk/glenns2634?_trksid=p4340.l2563
It sells all kinds of weird stuff at a good price so I definitely recommend checking that out. Remember you can buy stuff from eBay USA even if you aren't from the USA, as long as it delivers to your country. I have found that eBay USA have a larger range of items for snail mail and it is much cheaper.
The other places that I have been recommended to buy stationery from are:
I have only just tried Janet Store so expect a review on that soon! I have not personally used any of the other sites as of yet but I'm sure I will soon!
I hope that helps anyone looking for cool paper and stuff.