A lot of people bought theirs from
http://www.leeparts.com and that is where I got mine from. Also that is pretty much the lowest price to get one at, unless you find a used one on fleabay.
No one is going to agree on this subject, but IMO it basiclly boils down to this
Stillen-very good quality, but too quiet for some peoples' taste
Artisan-very good sound, but hard to find
Borla-very loud and very pricey for an axle-back.
Nismo-nice sound, good quality
The main reason I got the nismo was because i'm trying to keep with my theme. Which is (tastefull/quality/not too flashy/nissan/nismo) lol thats a pretty specific theme now that I see it written down