Hanlon motorsports sells improved T45 parts. What exactly are you wanting to do with the car? How much power?
I am running their kevlar syncros, improved shift forks and a basic rebuild kit and I am very happy with my T45. I bought the parts and had a local guy install them. It saved a bunch of money compared to shipping a 150lb transmission across the country!
Anyway, not everything Hanlon has is online. Give them a call and they will set you up with what you need.
As for what I recommend upgrading: definitely replace shift forks with stronger units, new syncros will keep everything working smoothly and a basic rebuild will do for most. If you plan to make BIG power, there are input shafts with more splines for increased torque capacity. The T45 is really a pretty good transmission... it just tends to break forks and syncros when you power shift it regularly.