Play Money General Information

Collecting play money may encompass almost anything that resembles money, but is not actual, legal money.

Lots of different ones including Lucky Play Money, Cracker Jack Coins, Mazuma Play Money, Pirate Treasure, Play Coins of The World, and many others.
Bills, Currency
Also many types including Lucky Bucks from old comic strips, some in store packages, money with toy cash registers, party favors, and carded money sets.
Game Play Money
Mostly with boxed board games.
Box Sets
Some of the nicest examples are found in the older sets from Milton Bradley, Parker Brothers, Ideal School Supply, and others.
Usually found in packages in stores or some older boxed sets.
All types of bills and coins that are used for various promotions.
Probably will be wrapped in foil. Much of it is holiday oriented.
Where Found 
Retail Stores
Wal-Mart, K-Mart, Dollar General, Big Lots, Toy stores, etc
School Supply Stores
Mostly newer sets for educational use. May be free-standing stores or in bookstores.
Antique Malls
And other antique type shops.
Old Sunday comic strips, especially from the 1930's and 1940's.
Collectible shows and meetings, such as SETS, FUN, etc.
Good sources are garage sales and flea markets.
Both online and mail bid: eBay is the largest online.
Collector type publications: Coin World, NTCA's Talkin' Tokens, etc.
Lakeshore Learning Materials
Newer sets; just search for "play money".
Web Sites 
Lots of colorful bills, mostly in large lots. An eBay store.
Lots of bills of all types; novelty notes, movie money, etc. Many pages at this site.
Play bills and coins, mostly newer types.
Richard Clothier, collector.
Search for "play money" or other items of interest.
The newest sets on a nice web page. (800) 477-7745.
More novelty notes.
Very professional notes.
An amusing site with many printable novelty notes.
Nothing for sale, but what a nice collection and great display method.
This is one of the few collecting fields which is still quite inexpensive and yet there are literally thousands of different items to collect. It can be as narrowly defined as say "TV characters", or, like my collection, it may include almost any type of play money. You are limited only by your imagination!

As an aside, "Ephemera, exploring the world of old paper" posted a nice article about the hobby: "An Interview with Play Money Collector Jack Phillips".