Hi Nathan,
I know this isn't helping, but I've worked in electronics for 30 years, and I don't remember ever seeing a 90 degree pot. Do you need it to physically stop at 90? Most "single turn" pots are about 270 degrees. If you just want a certain resistance range from 90 degrees of movement, you could try a 270 pot that was about 3 times larger resistance wise than the range you desire.
Wayne
ETA: I see now that is prolly what you were saying in your second post. Linear pot made me think "slide". Sorry