Seats built for Lotus cars such as the Elise, Exige, Elan, etc. are usually narrow enough to use in Cobras. Miata seats may be narrow enough as well.
Check out the chart from Flyin' Miata, which lists key dimensions for a variety of seats, though some may no longer be available as the chart was created some time ago.
https://www.flyinmiata.com/tech/seats.php
Another article worth checking out is from SL-C Wiki, as it focuses on seats to fit RCR's SL-C, which has similar seating width constraints to Cobras:
Seats — SL-C Wiki