Hello
I believe the part you are looking for is the plastic coupler cover 90606. This depend on the year your F7A was produced. Go to the Diagrams at
HOseeker.net to get the parts number.
I know Ed who has numerous Athearn Parts has a couple of these listed.
http://www.hoseeker.net/choochoo/athearntrainparts.htm However the shipping on these items will be very expensive.
You can try Horizon Hobbies at
www.Athearn.com
If you are coming to Houston there are a few Hobby Shops here that might carry the coupler housings.
PaPa Ben's on South Blvd off of Kirby Drive
G&G hobbies on Times Blvd off of Kirby Drive
Houston Roundhouse just off the 610 Loop South @ South Main Street
These shops are within a few miles of each other.
There is also hobby shops at these locations
Trains and Planes on Westheimer
Larry's Hobbies in North Houston off of I-45 at FM 1960
Spring Crossings in the Town of Spring off of I-45 about 25 miles North of Houston
Hope this helps
Larry