Pretty much any frame shop will have museum glass in stock or can order it. I have about 6 cases in stock right now.
If you are an end user who is in the business of selling your work, then you might be considered a business by the wholesalers and can buy it from them.
Usually, you need to have an actual storefront or proof of an actual business such as a resale tax number before they will deliver to you.
Distributors vary on what they want to see as proof of business.