Stillen is one of the best. Although they sell other manf parts. Jsut go to their site and try to find the best price. you can go to
Summit Racing and find the parts there. I don't know if it's cheaper.
Bypass valve? For an inside engine air filter? Don't think so!

You only need a bypass valve if your air filter is close to the ground (cold air).
I know they have different models. Just choose the one you want to go with.
BTW: get some underdrive pulleys while you're at it! you'll thank me later.
-edit-
I second the exhaust suggestion. that'll give you quite a bit more performance gain if you combine it with the other mods. how much you willing to spend?