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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.coins, 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.
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