For in-stock keycaps that aren't too expensive, try Cannonkeys, or Novelkeys when they have a sale. You can also check thocstock. Drop is also an option, although nobody likes Drop.
The coolest keycaps are generally sold as group buys run by dozens of different vendors around the globe. You can check mechgroupbuys or keycaplendar for info on those. (Or browse geekhack)