This article is from the Paper Money Collecting FAQ, by Bruce Giese firstname.lastname@example.org with numerous contributions by others.
Your best bet is to go through a dealer or another
avid collector. You can also keep whatever is left
in your wallet after a trip to another country, but
you'll end up with a very limited set of notes in
Most countries have notes that cost less than 1 or 2
US dollars. Countries with rampant inflation will have
a lot of notes from previous years which are very cheap
(South America, Yugoslavian countries, Eastern Europe).
Dealers get their notes from contacts within the
countries who buy CU packs (crisp uncirculated). The
world being what it is, often dealers take big losses
when contacts in remote countries essentially take the
money and run (I've heard of a major dealer who took a
big loss in Mongolia recently). Some countries don't
allow currency to be taken out of the country, so people
have to risk punishment to get the notes (e.g. Mauritania).
Doing business wholesale is tricky, but if you travel to a
remote part of the world, you can sometimes get dealers
to pay you to pick up CU packs.