I think Rodrigo sold that kit to lennySE-R, but maybe he has another one?
Yes, it's best to fix the problem on your current transmission. Buying another transmission will likely get you one with 5th gear pop-out waiting to happen again.
You can buy the entire kit (from Courtesy or Gspec; they are different kits) or you can just buy some of the parts you need to try and save some money (the kits come with some extras).
You can read up on what parts come with which kit, how to replace it yourself, and tons more info here:
http://www.sr20-forum.com/general-maintenance/18390-how-recognise-repair-5th-gear-pop-out-b13-transmission.html
I've replaced enough 5th gears now for people that I could do it in my sleep.