I learned a lot from www.aircooled.net as far as building a high performance engine. Go check out their help section. Great site, great guy, great parts.
It is the most informative site I have been to as far as parts information etc.
After soaking up all the info you can from their, I'd say go to www.cbperformance.com for more parts.
These are the two aftermarket sites I'd say that I trust the most. Good reps and great people.