Often times, you dont require a replacement gasket. Take it apart carefully. If it doesnt tear, you're fine. If you want one, go to your local repair shop. They should have them.
Thanks for the advice. I know it's in the carburetor because when I give it throttle at the carb there is slack in the cable to the pedal but it still doesn't let off. Where would I get a new gasket for something like this once I take the carb off to clean it?
Sounds like its either the throttle cable itself, or the throttle butterfly valve. the butterfly can be cleaned with carb cleaner or something similar, they get pretty dirty especially on a unit that old. If its the cable, spray some chain/cable lube into the cable at each end and hit the pedal a few times to work it in