Inner tube advice needed

[walkerlakeprinter]

Do any of the suppliers sell a heavy-duty 21 inch inner tube? I patched, installed, re-patched, installed my older one....another leak! Gave up. Got my new one installed today......new leak. Patched, installed.....still leaked around my new patch after I made sure it didn't leak before I put it in. I was careful to not pinch the tube while remounting the tire. I have had no leaks in the four tires on the car, and they have been holding air just fine for at least 21 years!