New 5spd tranny for 2003

I think I am going to need to replace my trans soon. It makes a great deal of sound in all the forward gears except 4. It also makes a lot of noise parked in neutral idleling. I have had this trans rebuilt before and wondered where/if I could get a 6 spd for it and how much if any mods I will have to make to get one.
What is a good aftermarket trans.