I do! I don't buy them, though, I've just saved notes and coins from places I've travelled. I think of it more as a way to remember those trips than as a collection of objects--they're keepsakes, not purchases.
If you feel differently, however, and you're interested in getting your hands on some foreign cash as a collectible, you might want to try going to a major bank branch. Main branches of banks in major cities will often have a small amount of foreign currencies on hand for people who want to get a bit of cash before travelling. They're likely to have euros, British pounds, Japanese yen, and Chinese yuan, but they might have some other ones, too.
It's really interesting to see what kind of designs different countries put on their money, and how the shapes and sizes are different from ours. For instance, euros are shaped differently according to their value--a 20 euro note is larger than a 5 euro note. While that means that they don't all line up exactly in your wallet, it makes it a lot easier for blind people to handle their own money, which I think is a great idea!