Instructions
Purchase a Monster Vault or similar product online or from a retail store. The Monster Vault, available at hobby and gaming stores as well as from online stores like Wizards of the Coast, contains a large number of monster tokens. However, the tokens are more or less random, so if you want or need specific monsters you may or may not get them in your Vault.
Purchase smaller sets of Monster Tokens from Wizards of the Coast. The company offers smaller sets, similar to the Dungeons and Dragons Miniatures game that ran for a time, where you can buy a set of random tokens. You get fewer than the Monster Vault, but the sets are less expensive. They're still randomly packaged, however.
Check on auction sites like Ebay.com for specific monster tokens or groups of monster tokens. Good deals on gaming gear like tokens and miniatures can be found on auction sites, especially when gamers want to unload unnecessary equipment or gear they don't use.