The 80cc shifters are a little trickier to find but they are usually pretty reasonable. Maybe a little less supported but like you said, they are dirt bike engines and Honda/Yamaha/Suzuki/etc aren’t really going anywhere soon. I’d be more careful with the chassis so you don’t find yourself with something that parts are hard to find.
I believe there is a shop at Atlanta Motorsports Park. Joey at the Carolina Motorsports Track is a great resource as are the guys at KartSport in Mooresville, NC.
Might find it easier to find a 125 shifter – they are more plentiful. Most of us will tell you that shifters aren’t a good starting place but if you are just going out to have fun more so than race you should be fine. If you have general mechanical aptitude you’ll figure out the basics in a hurry.
Can also check out kartsale and kartingsource for additional equipment for sale but here at EKN is always the most active marketplace in karting.