Rickettsias are apparantly the closest related prokaryotic group to mitochondria. Not practical to use though.Bacterial colonies would be my first choice (although they don't have mitochondria, but mitochondria are symbiont bacteria so some bacteria would probably have the same ability).
You could choose a relatively simple eukaryote to preserve the mitochondria. It could still be unicellular and exist in completely controlled conditions.
Heh. Yeah. Maybe someday a few people in a micro lab with access to a mass spec will do it as a hobby. They sure wouldn't get any support for it.This would take the talents of a really meticulous microbiologist plus a very good instrumental chemist, with clean facilities and an administrative environment that did not throw them out on their arses for even bringing it up.