I picked up an air pump today, and it didn’t come with check valves. Surprisingly, the check valves cost as much as the pump did. Instead, I built a bottom cap assembly using an old air valve from a bicycle, and so it ended up costing me nothing, and getting me a really nice looking bottom cap assembly at the same time.
I wrote a description of how to modify the high-pressure check valve in the bike valve to a low-pressure one that suits our needs, on my blog.
Hopefully this will benefit someone else