Speed inc. sell's them. You could find a nice k&n. rev extreme makes a real nice one too.

 

what does a oil cap breather do for your engine

 

It is a filtered opening in one of the valve covers (which on our cars there is really one good one and that is where you put your oil. There is another but it is extremely tiny and not worth it) and it allows the PCV to draw filtered air through the crankcase to relieve crankcase pressure.

I dont see it really being beneficial (though I dont think it would hurt) to low modified/stock engines, but with bigger forged engines it is pretty much a gimmie.