On Thursday, April 28, 2016 at 6:59:32 AM UTC-4, Ignoramus31353 wrote:
I sold a customer this hoist, and, unfortunately, upon delivery the
sight glass was broken.

Here are some pictures:

http://igor.chudov.com/tmp/Sight-Glass/

He says that it is a round glass with a little groove for a o-ring.

My question is, where can I buy him a replacement glass, any idea?
McMaster seems to have complete assemblies only?

If you can get dimensions from him, and have him send a sample so you know what the o-ring groove looks like (he should send the o-ring as well), any optician should be able to bang one out for you. That's pretty much what they do all day.