Shall we understand that you can't buy any tires at your local hobby shop, or you are not satisfied with what they offer?
Perhaps you could tell us what surface you are racing on and what tires you have tried yet. That would be a good point to start from.
Another avenue to be explored is to look at what the other people you race against are using.
It is also important to find out if you don't know yet if your club/race track has a control tire?
There are too many options to think about without some specific information.
Team Greasy Weasel
The best upgrade to any car is some driver skill.