This article is from the Paper Money Collecting FAQ, by Bruce Giese email@example.com with numerous contributions by others.
The most effective way to get paper money is not through
general circulation, but through dealers, collectors, auctions,
and shows. About once a week someone posts a message asking for
people to trade/buy/sell paper money. Even with the large audience
of rec.collecting.paper-money, you don't reach many collectors. But this
is still one way many people get notes.
The International Bank Note Society's quarterly journal carries
free advertising "of a non-commercial nature" for people wanting to
update their personal collections. Bank Note Reporter also has a lot
of small cheap ads for this. These are good places to look for people
to trade notes.
Another way to buy/sell notes is through mail auctions. The IBNS
has frequent auctions and lots of individuals have their own (which
are advertised in BNR).
But I find the most effective, painless, and sure fire way to buy
and sell notes is through dealers. Dealers range from people who
just have large collections to those who keep a large stock of notes
for the specific purpose of selling them. In any case, dealers have
developed the methods which are most effective for making collectors
happy with their transactions. Some are notoriously grumpy, but most
are truly friendly and helpful.
Some people prefer to collect notes from general circulation. This
is a good way for someone to start getting interested in paper money,
but it's really not a very effective way to collect it.