4x5"/9x12cm enlargers and lenses are plentiful and most work just like enlargers for smaller formats, and can in fact be used for smaller formats. If you stick to a popular brand like Omega, Beseler, or Durst, you'll find lots of parts and accessories available for them.
Adams had horizontal enlargers for printing big from 8x10" negs. Don't worry about that. You don't need a horizontal enlarger for 9x12cm unless you are making murals.