If you choose to order a new dome light assembly, it looks like the part number is Subaru 84621CA000VH. This will replace the entire assembly, including the outer cover, lens, and bulb. I don't think you can buy the outer cover separately from the lens and bulb.
For future reference, here is where I located the part - all of the parts are visible using this online catalog and others like it.
https://www.subarupartsdepot.com/oem-subaru-parts.html
If you have a Toyota GT86, you may wish to search a comparable online Toyota catalog - they use their own part numbers. For this particular part, I believe the Subaru part is interchangeable.