In Your Pockets
Start by searching your spare change. One of the more fun aspects of collecting can be the joy of finding something after daily searches. Keep an eye on the quarters that pass through your hands as you do your daily business. As millions were made of each coin, there are a lot of them still in circulation.
Coin Collectors
Purchase the entire set. Several coin collecting sites will sell you a collection of all 50 state quarters. Determine whether you want one quarter from both mints or whether you just want one quarter per state. Some online sites include Continental Mint, The Franklin Mint and even Amazon.com. One site even sells them set in collectible spoons. Many of these collections are sold in special folders or are in a framed display. The extensiveness of the display will figure into the cost of the purchase. You can also purchase complete sets from local coin collector stores.
Auction Sites
Purchase individual coins. If you are looking for specific quarters in the series to round out your collection, consider turning to auction and classified-listing sites. You may be able to find individual quarters on eBay, or post your request on a site like Craigslist or Freeservers.
Friends and Collectors
Ask your friends and associates to collect the quarters with you. When you get together with a group of collectors, you can trade quarters that you are missing from your set. This way your collection grows and you enjoy social time with others.