You buy the one appropriate for the scale/type RC you have. They are great. I like to use an outerwears filter cover to prevent the cap from coming off. It does allow more air for the engine to breathe in so tuning may or may not be need to compensate.
i bought the one that tower said was for a revo looks like it should fit adn where do i get the outerwears i have them on my quads but didnt know they made rc ones.
and she was running way rich today so i probably shouldnt have to retune
I would say that you should have to retune, but you most likely have to adjust your tune each time you run (if you're running a proper tune) so it shouldn't be any different. What I'm trying to say is if I run my RC today, even though I adjust the tune throughout the day with changing weather, when I go out tomorrow, I'm definitely going to need to retune. So with or without the filter I'm going to adjust the tune, so it shouldn't matter much.