Just because someone has an image that is in the public domain doesn't mean they're obligated to share it with you. They can simply say no. Your options are:

1. Hire a photographer to take a photo of the artwork that you want to use. You have to pay the photographer, and you still need the museum's permission to access the work in a setting where you can take a good photo suitable for publication. You probably can't just snap a photo while touring the museum and use that.

2. Use a photo that the museum has and pay them and/or agree to their terms. Maybe you could take that photo and then share it because it's technically public domain, but guess what's going to happen the next time you want to use one of their photos then?