I have a '93, I'll assume the airboxes are the same and try to help.
There are 4 screws, one on each side of the box - have you gotten to all of them? If they turn but don't come out, the piece they thread into may have fallen out - this happened to one of mine. Try prying it open with a screwdriver and just lifting it off - without breaking it obviously.
Finding a replacement air filter should be pretty easy - STP # SA3717 is what I use, in stock at my local AZ. No one makes a cold air it for these cars, you're on your own so to speak but a few guys have fabricated one out of PVC pipe or some 5.0L pieces. Google or the search function on this site should help.
Good luck!