If your just looking for a sparky look at www.bmp.motox.org under yz400, and yes it fits a yz250f. My buddy got one for his 250f and it works great. It increases the backpressure a little which increases the low end grunt and takes away a just little on top. It still breathes well and you don't have to change the jetting. It also makes it a little quieter, not much. They will also ship it with the fmf endcap.
I put the Big Gun on my 400f (Oversize quiet series with quiet insert). It is extremely quiet, 84 db at idle and about 99-100 db at 1/2 throttle. It also breathes well, you'll have to increase your jetting to correspond with the added airflow over stock. I haven't seen/heard one on a 250f though. I'm extremely happy with mine.