Instructions
Go a a local coin shop or dealer. These places have many types of coins and should have a mint Mercury dime without a mark that can be purchased.
Attend coins shows that are coming to your local area. Coin shows attract all kinds of coin enthusiasts and are a good place to find all types of coins, including Mercury dimes without a mark.
Look on eBay to see what sellers have listed. A mint Mercury dime might be found there.
Use Craigslist to see what people have to offer. Sometimes, collectors will sell all or a portion of their inventory.
Post comments on various forums such as Coin Talk, (see link in Resources). An online forum is a good place to find enthusiasts who may be looking for the same type of coin.