There are other sources for high UV transmitting glass other than copier glass.

What people want to look for is 'low iron' glass. This glass is sometimes used as framing glass because it looks very clear, without the greenish tinge of normal glass. 'Water White' is one trademarked name, by Denglas.

My information searches in the past indicated that typical low iron glass has about 1/3 the UV blocking of normal (borosilicate) glass. The difference is not great in the final exposure time, so I wouldn't go out and replace all your glass unless you have a reason to, but if you do need to, you may think about getting low iron glass.