eBay for the parts, and unfortunately you have effectively ruined the wheel by honing it. The splined section of the axle shaft is not actually part of the axle shaft, it's a bushing that's supposed to be pressed into the wheel (the splines keep the bushing from spinning inside the wheel.) The bushing is supposed to spin freely on the axle shaft, but in your case it's seized (likely rusted) to the shaft. Proper way to fix this would be pull the wheel off, un-seize and remove the bushing from the mower, and then press the bushing back into the wheel and reinstall everything.
The reason that I say the wheel is ruined is that the splines on the bushing will grind away at the plastic of the wheel and enlarge the center hole. Very soon, the wheel will be so wobbly that you won't be able to push the mower. Only way to fix this PROPERLY is to get a new wheel.