I believe Summit Racing has them. I will need to be looking for 4 lug rims soon also. Let us know if you find any good places.
If you are looking for used wheel, the late 70 & 80's Nissan 260-280z cars will fit. Sometimes you can find some in 15 inch. to the forum. Rick
Swap meets and ebay are good places to find 4-lug wheels. Make sure you get the right bolt pattern. Pinto, Mustang II, and Fox body Mustangs 4-lug is not the same as a Maverick. Falcon and Early Mustang 4-lug would be the same as a Maverick.