I used just a generic chrome base from the local parts store. I can get you the measurements if you’d like?

 

I am guessing you have an older afb with the smaller airhorn. The newer afb can use the generic 5 1/8" base. Good luck finding one for the smaller ones. You might try a early maxwedge repop aircleaner, or have a spacer made to use a generic base.
A little more info about which car, and which carb, would be helpful.

 

Early AFB has a 4 7/8'' air horn. I believe 67 was the last year before they went to 5 1/8'' in 68-up.You may try Edelbrock's web for a adapter then use a cheapie 5 1/8'' base. In the photos I use the top of the early cars. Slants and Commando's were similar but not chrome like the Commando. This was a 6'' in the photo(what I had at the time) a 9'' base works better.

 

9'' base with a Slant top with a adapter to a small 4 7/8'' AFB.64 Plymouth

 

If you find a 360 2bbl air cleaner from 72 to about 89 you can make your own base out of one as the 360 2 bbl is a 4 7/8'' air horn. Even 383/400 2bbl work too.
Cut to fit and use a 9'' air filter. No adapter needed.

 

Use this with a newer base...

Mr Gasket 2082 Air Cleaner Adapter.