I'd search eBay and Amazon for "PCP hand pump". Try to find one that looks similar, then look for it's applicable rebuild kit.Or you could just disassemble it (take notes and photos), then take the o-rings to a hardware store and match the sizes.
...but don't see any for the plain old cheap Chinese pump. Was hoping someone here had a "source".
After taking pump apart to get to o rings and driving to Lowes, Napa, Ace hardware, Oreilly auto parts, Advance auto parts and Southern plumbing supply I got some of the orings... Was not able to get ANY of the green o rings in the proper size. So I put it together with a clean and (silicone) lube replacing the o rings I was able to find. Tried it out and now won't work at all. Ordered a new one from ebay yesterday! For $41 delivered it's not worth another minute of my time trying to fix this one...