What are some good web sights for aftermarket products on 2000 and up sentras????

There are a few..

www.importpartsplus.com
www.evoracing.com
www.placeracing.com
www.hotshot.com
www.stromung.com (they make a really good cat-back exhaust,Full stainless steel,mandrel bent tubing,and 2.25 inch tubing..)
www.jimwolftechnology.com

Those are just a few of the sites but you should also check out some of the Sentra specific forums and websites...Like www.B15Sentra.net that is specificly designed for the 2000-2002 Sentra's...I am a modertor there....But here's a list of good informative Sentra sites....

www.B15Sentra.net
www.Sentra.net
www.SR20DEforum.com
www.SE-R.net

Hope this helps...