Most auto parts stores. Motorcycle shops also carry it. Eastwood also has a good tank sealing product.
I would take the tank off, remove the sender, drain, cap, etc and pressure wash it. Some of the old timers swear that strapping a tank to a rear tractor tire, putting a couple pounds of old nuts/bolts in it and driving for 20-30 min cleans up the inside rust. Probably does.
Then use a good tank sealer. One caution, The tank inside needs to be oil, grease, fuel free. I would buy a gal of acetone and after cleaning slosh that around in the tank. Then spray a couple cans of brake cleaner in it. Then use the procedure in the seal kit.
Oh, some better radiator shops will seal tanks too. Cost is about $100 over the kit price on average. Usually a good deal if you can find them around your house.