Do you mean to explore? All the sheet film sizes of chrome ('slide") film are disappearing except for Kodak Ektachrome 100; so that's the key one; and it's excellent. But Fujichrome Provia 100F and Velvia 100 can still sometimes be found as options. It might be hard to find any of them in 4x5 at the moment; but production goes in cycles, and at least Ektachrome should be available again pretty soon. In the LA area, check with Freestyle and Samy's.
Do you mean to explore? All the sheet film sizes of chrome ('slide") film are disappearing except for Kodak Ektachrome 100; so that's the key one; and it's excellent. But Fujichrome Provia 100F and Velvia 100 can still sometimes be found as options. It might be hard to find any of them in 4x5 at the moment; but production goes in cycles, and at least Ektachrome should be available again pretty soon. In the LA area, check with Freestyle and Samy's.
Either way, consuming 4x5 chrome film is getting expensive; but at least it's nothing like 8X10! But if he's fooling around with cross-processing, there are some deals out there on outdated chrome film of various types.