Welcome to the forum!
I am assuming that you are referring to a 2-stroke engine. Just about all the vendors that advertise on this site sell piston rings.
As for a "piston ring clamp" I think you mean a piston ring compressor. They are not really needed on these tiny little engines but if you really want to buy one you can find them at auto parts stores, Sears, some K-marts, Tractor Supply, ect. Just be sure to get one labeled "for small engines".
FWIW, upon installing the cylinder, the most common reason that rings get broken on 2-stroke engines is the ring end gaps are not lined up with the locating dowels that are pressed into the piston ring grooves of the piston. If the little pin is not in between the ring ends then the ring cannot compress into the piston groove and *snap* awww darn it! Don't be afraid to use pre-mix oil as an assembly lube either, You want the piston, rings and cylinder wet with oil to aid in everything sliding together easily.
Take note that I said wet with oil and not dripping all over with oil. There is no need to make a mess of things for yourself.