Sparks makes slip-ons but they are about $425 but I would say get the slip-on right now because without the head work done or higher compression its just gonna rob horse power because the aftermarket header is designed too flow more gases than the stocker. Most likely you would lose a good amount of power over it, but if you are doing head work right after u buy it then thats a different story. I'd say HMF or Pro Circuit if your looking for a pretty good sound and alot of low end to midrange power and pretty good top end, but if you are looking to get a really good midrange hit I would go with a White Brothers e-series and its got a great sound too it.
2004 450r
-Renthal Bars
-Polished HMF Slip-on(Eliptical)
-K&N filter w/outerwear
-Dyno Jet Kit