You should be able to get one for around 100K USD.
You asked for the best
There are a few options, from what I understad the stock (JDM WRX) subaru trannies don't have problems with the EJ20s because they don't produce as much gear-crashing torque as the EJ25s.
Your best bet is to do a search in this forum, I'm sure this has been covered a few times. And don't forget, while there are some differences, the US EJ20 and the JDM versions, it won't affect your transmission choice.