Viton O-Rings
I purchased some viton O-rings from an automotive shop which did a lot of transmission work. Check your local auto parts store. They may have them.
You don't HAVE to use Viton rings, but they may last longer than the neoprene rings. You might just purchase some neoprene rings (they're cheaper) and try not to get solvent on them when cleaning the gun.
BTW, be sure to take the existing O-ring with you when you go shopping to compare and be sure you get the right size and thickness.
