You can look on line and in pawn shops but you are more likely to find these and other rare coins directly from local collectors. Keep in mind that as with other rare coins the 1945 dime or mercury head dime as it is sometimes known can vary widely in value based on condition.
The value on these semi-rare coins vary depending on the condition of the individual coin. The low end are coins in good condition and the value on them is strongly based on the silver in the dime, it can be expected to fluctuate with the price of silver. The high-end coins have not been in circulation and still have the shine from the mint on them.