Heavy Throttle is a good place. I have not bought anything from them, but I have e-mailed them before. They are very prompt in answering any questions you have, and know lots about all of their products.
Unstable-Hybrids
http://www.unstable-hybrids.com is another company that is very well known in the business.
Those are both on the east coast (I think). One in GA and one in NY (I'm pretty sure).
As oberkill said, there is a place called Boost Dynamics
http://www.boostdynamics.com/ in the mid-west. Again, I have not bought anything from them, but I have e-mailed them, and they are a good group of people. Probably the only reason why they aren't as well known as some of the other guys is because they are in the mid-west.
There's a place in TN called DA Performance
http://dave_240sx.tripod.com their prices are great, the best out there, but the engines really do seem to be missing a little bit more than most places. If you go through them, ask tons of questions about everything that comes with.
Then in CA, you have
http://mckinneymotorsports.com and they are very well known on the west coast. I have heard nothing but good things about them, but have not dealt with them at all.
There you go, there's a company for every part of the country. Enjoy!
BTW, I am trying to set up a group buy for SR20DET's, and all of these companies have been contacted (except for McKinney, which I might go ahead and do). Let me know if you might be interested in participating in that.