Oh man, that's the classic K1000 lesson! It's almost a rite of passage to learn it the hard way. That main advance gear is so fragile, and forcing it just puts all the stress on the weakest point. I've had luck finding donor parts from broken cameras listed as "for parts only" on auction sites. You have to buy a whole junker body sometimes, but it's often cheaper than hunting for the single gear.
Yeah, buying a whole junker body is the move. I did that when my film rewind crank snapped. It was cheaper than finding the part alone, and I ended up with extra screws and springs for later.