First thing, what you have is a common business strike from Philadelphia. The coin you refer to is a proof struck in san Francisco that did not get an S mint mark punched into it. Search for Proof coins on your favorite search engine to see what a proof looks like and you will see that what you have is nowhere close to being a 1968 no S proof dime.
About the only chance of finding one is from proof sets that haven't been checked. You are not going to find one in circulation.