You'll need to replace the entire light. However there are frequently people selling used fogs in good condition either separately or in pairs so you should be able to find a replacement for far less than the dealer charges.
Check here in the "OEM Parts Pit" forum, also post a request in the Wanted forum.
Try over on NASIOC also.

Once you have a good set of fogs again, I strongly recommend applying a protective covering to them such as Xpel, Stongard, etc. to prevent this from happening again.