Usually they are built into the engine. I haven't heard of a pro that puts one of those $10 dollar ones from the small companies.
They really have no positive effect. Theoretically they should work, but, they are too small to give much of a boost in engine performance (pun not implied). Some people say they work, but I heard the late Ron Paris
say that they are not really worth it because you could much more easily gain power in other areas. They are eye candy in my opinion. ( you could try tell neighborhood friends that it is a nitrous kit for your engine
. Some people believe anything.)