Pretty much anything made for Leica M mount can be used with either the official Fuji M Mount adapter, or 3rd party versions
So you have a wide range of manufacturers in that field, and a massive pool of used kit to choose from.
All manual focus etc of course.
Leica lenses are of course pricey, but you have all of the 3rd party M mount manufacturers like Voigtlander at nicer prices
You can also find adapters for most lens makes on EBAY or Amazon - not sure how compatible they really are, you'd need to research
For example, you can get some Canon EF adapters, which work, but you can't change the aperture on the lens because Canon aperture is changed electronically! DOH!