I don't know how much you can get around but if your parents could take you to a coin show you may find lots and lots of
Mercury dimes for really cheap prices. Also, you may want to check via the internet for any coin stores in your area. For many of the more common dates they shouldn't cost to much at a coin store.
Some members here to may sell you some for a reasonable price. I really think you should avoid
ebay and/or other on line places untill you really know what your doing. Many coins on the internet you really can't see until it might be to late.
The chances of finding a
Mercury dime in rolls or bags from the banks are really slim. Way to many people already searched them all.
I've found fantastic deals at flea markets so if you could convince your parents to take you to some, there too you may find some of those. Not sure how old you are nor where your at but another place to find
Mercury dimes and other coins is at garage/yard/estate sales. The trick there is you have to ASK. As a rule people don't put coins out for those types of sales due to the massive amount of unknown individuals that stop by. But if you ASK, they may actually have some old coins you can buy. One of my greatest finds was at a garage sale. I ASKED and a lady said we have an old jar of pennies that we were just about to take to the bank. They showed it to me. I estimated abaout $35 dollars worth and offered $50. The entire bottom of the jar was all
Indian Head cents. No coin was newer than in the 1950's.
The thing is you have to ASK.