Where do I find replacement parts?

You may have no choice but to use one or more "fancy tools" to remove them (depending on their condition; since they've apparently become brittle, the other one might break off in a similar way. The front wheelset has a separate bushing bolted onto one side of the axle, while the driving wheels are fitted (very) tightly and their axle has to be pushed out somehow. Here's an existing solution for the drivers. Remember: if you replace one wheel and not the whole powered wheelset, you'll have to repeat the same process/es on the donor.