Generally, if the key is worn out, the hardware store/dime-store people are not going to be able to "fix" the issue no matter how many keys they cut.
I think you have the correct solution, take your old key to an automotive locksmith and let him decode what you have and code cut a new one.
Generally (on the older locks), the key will wear out before the lock internals. If you really want to do it (repair) to last a while, have at least the ignition and driver's side door locks re-pinned. It's not just the pins that wear or gum up, but the springs will crap out. The greatest/newest pins are worthless if the springs are shot. The locks are also subjected to whatever POs have sprayed in them as well as what may have been picked up from a pocket. I have seen a lot of crazy crap inside a lock.
I would also agree that rebuilding older locks (OEM) is a better option than buying "new" Taiwanese / Chinese junk. I have bought them and had them malfunction within a year. The only caveat is to make sure the locksmith you take them to has experience in older car locks. If they don't, they may not have the parts/knowledge to correctly fix them. Another plus is they have the older pinning kits which are a better quality. I have stuff in the basement that I have had 35+ years that would be hard to replace.
You can pull your locks and take them in. (recommended) Benchwork is much cheaper than the alternative. If the lock is worn out, there isn't much they can do with it in the dash.
Get a new key code cut. Take some electrical tuner cleaner and clean out your locks. Flood them, but be careful to keep stuff off the painted surfaces. (another plus to pulling them). Dry with compressed air and squeeze in some dry graphite.
