you can for Boethius. I'm not posting a link as it's still in copyright but you can find a pdf: add "bower" and "fundamentals of music" to the terms in a search engine. you will find it's available at a certain archive site.
frankly, you'd be better off with "the critical nexus" by Charles Atkinson. the cheapest online/ebook option is through google play. imo, it's one of the best books on how early western music theory developed.
but, frankly, if you're a beginner as stated in the OP, I'd get to grips with some more modern music concepts and skip all the ancient modes stuff for the moment. you've started with a bunch of ill-conceived premises and it's all spiralled out of control from there.