My lab (unfortunately long closed) used to make all it's internegs by enlargement on to sheet film, however they had the best copying set up available, essentially a De Vere 5x4 head used as a light source with a copy camera above it. You do see these for sale sometimes.

I have to disagree with Bob here as this gave the highest possible quality internegs, it was also used to make 5x4 transparencies from 35mm Kodachromes etc.