I believe they're 44mm and called #212? You'll need 3. You should pop the lense cover off to see what LED fits best. There's lots of options out there.
I've bought many LED's and bulbs from the folks at Autolumination. Here's a link to their festoon type bulbs: Festoon Dome LED Lights Bulbs | Autolumination
I had the Mustang in mine for a long while.....but the my stock "bubble" dome kicks it's tail in performance IMO. And it takes 2 wedge based bulbs and one 44mm festoon.