I am trying to find a solid core interior door that does not off-gas formaldehyde? Any suggestions?

wood?

I was only being partly flippant with that reply. The options include a solid wood core, gypsum core and there is a manufacturer that offers an agfiber core called Greencor 20. I have not found a door commonly listed anywhere that uses this core but the AWI should have a list such manufacturers exist yet.

Gypsum core is a great choice becasue it has a unbeatable fire rating as well as STI rating (sound).

Good hunting.