congrats on buyin a mk3
well you can buy an air filter from just about anywhere... just find your local performance shop or automotive accessories shop and they should have an air filter and adapter there
adjustable suspension eh.. hehe.. i'm assuming you are talking about the factory tems? or have you got aftermarket suspension in there?
you can lower the car with tems... doesn't bother it too much i reckon.. i had mine lowered 50mm but when jacked up the springs aren't captive.. on the ground they're sweet tho
i got my springs compressed rather than buying aftermarket ones
good luck and have fun with your new supra