I don't know of a specific brand to go with that wouldn't turn yellow but I wanted to point out that it is sunlight that does that. The UV rays in sunlight cause the plastic to discolor so, if possible, maybe try parking out of the sun when you can?
Maybe you already knew this but I have seen cars that parked regularly in such a way that half the car was shaded half was in sunlight, and the difference over time in the two headlights was night and day as far as to how badly (or not) they yellowed.
I know your pain as that yellowing drives me crazy as well....
Good luck!