Here's a few that I know of. I haven't order from most of them so I really don't know that much about them.
http://www.jetboatperformance.com/frame.htm
http://www.eddiemarine.com/
http://www.boatheaders.com/index.htm (this is Lightning Performance Marine)
Here's a few non jet/performance sites
http://www.surplusunlimited.com/
http://www.boatingamerica.com/superstore/index.html
http://i-netmarine.com/index.html
http://www.overtons.com/
http://www.skilimited.com/
http://www.championtrailers.com/
Good Luck!
If your more specific as to what your looking for I'm sure the guys here can help you out.