Yeah, I actually just double checked and turns out you can buy or stream the Realta Nua Soundtrack on Amazon!

Link: https://www.amazon.com/Fate-night-Realta-Soundtrack-Reproduction/dp/B099HX8XMM

But luckily, all Type-Moon soundtracks aren't too hard to find on Youtube. I've been able to find and listen to Fate/Stay Night, Hollow Ataraxia, Mahayo, and Tsukihime/Tsukihime Remake fairly easily.