I think USPS will only accept stamp orders from outside the US via telephone. They do have an eBay store though where they will ship worldwide.

As eventar5 said, sending currency usually works too. I do that too, all the time! Still, including stamps increases a lot your chances of getting a reply! Good luck. :D

You can buy postage for the UK and Canada here. I'm from the USA, but if you aren't then you can buy our stamps online as well. If I'm sending to any other country, I just send a few US dollars and that usually works for me! Good luck!

http://www.royalmail.com/personal/stamps-collectibles-gifts
http://www.canadapost.ca/cpo/mc/personal/productsservices/send/postagestamps.jsf
https://store.usps.com/store/browse/category.jsp?categoryId=buy-stamps

Hello, I was wondering if there's a way of buying stamps from abroad through a website or if you can only buy them through people.
Greatly appreciated