Instructions
Approach a coin dealer in your area that sells Buffalo Nickels. These are a commonly sought coin, but they are still in plentiful supply, depending on the year and condition, so you will be able to find one for a reasonable rate. If you are just looking for one as a curiosity, for example, you can get one from the 1920s or 1930s in "Good" condition (many signs of wear and tear) for $4 or $5.
Go to coin shows to look for more specialized Buffalo Nickels. As you move away from the coins that have a lot of wear on them and into the higher grades, numbered 60 and above, your price will go up. Online coin dealers may also be able to connect you with the rarer and finer surviving coins.
Look on auction sites for Buffalo Nickels. eBay is just one place that will offer coins -- and sellers aren't always as cognizant on those sites as coin dealers are; often, they are people who came across a box of coins in the attic and decided to sell them, and you can get them for far less than they are worth. Consult a dealer or reference before bidding, though.