How do I find a replacement for this light fixture enclosure?
Firstly you could take the fixture down, examine all markings and google search any words or numbers you find on them.

If that doesn't work, and you have a picture of it or can take one of another similar fixture, you could use TinEye to find a similar photo and hope that it's on a product listing page.

Replacing a light fixture however, is pretty easy.  I recommend buying spare globes for any glass fixtures and keeping them in a safe place.