Depends on what you're looking for, if it's wooden hideys then buying locally is better - postage prices for wooden toys would be pretty costly, I know my pigs wooden huts/log cabins are pretty heavy.
For plastic pigloos etc. most pet shops would stock them, or you could try eBay. I don't know of any "well-known" online guinea pig stores that sell them, unfortunately.
You could also try local pound shops for the plastic step stools, the one with four legs and open space underneath. Cheap and cheerful!