I had a iPhone 8 cross my desk this week with a broken glass in the back . Ive been looking for replacement glass and as of this week was unable to find any yet but am sure there will be some out in the near future . I think looking at the one that's here on my desk that the lens can be replaced only with a new back . The back probably wont come cheap as the charging system is integrated into the back cover and the glass is apparently very difficult to separate from the back plate so I'm guessing this will end up being a unit to replace . Don't despair though it wont be long till our friends in China have a replacement part for it. I used the ifixit teardown guide
[guide|97481]
Hope this helps