O-rings are sized by ID and thickness. If you get some calipers, take some measurements, post them up here, along with photos, maybe someone can help you? I do a lot of O-ring work for different products, and have some understanding on how to size them. The issue you will have, though, is finding a place to sell you just one or two. Industrial supply houses don't make money that way, so you may need to buy a pack.

Regarding serviceability, that costs money. A pump will last for a good long time on the original seal, and there is not much of a business case for a manufacturer to stock service parts for a cheap pump. Even parts like Shimano are either hard to find, or non existant after some years. No sense in fighting it.