Listed in your owner's manual. Don't have one, then get one. Sources are Dealer, Salvage Yard, E-Bay, and many on-line sources. Google search.
I have never come across a 1999 Explorer that had a cabin air filter. I am looking at the Ford online service manual right now, and I don't see a listing for it, nor do I see a picture of it anywhere in the diagram. All indications are that vehicle never came equipped with one. My 1999 Ford Explorer Does in deed have a cabin filter, it is located behind the glove compartment.