so for some reason i am unable to access a summoned Golden Saint's inventory immediately after i kill them for a Daedric tower shield, even though i start to spam the use key when their health gets low. since this method doesn't seem to work, can anyone list some locations where Golden Saints tend to spawn?

The grazelands, some parts of Molag amur, various Daedric ruins. I'd try for Daedric ruins though, especially Ald Daedroth.

This is really an exploit, so you might not want to try, but if you go to Tel Vos (Home of the Telvanni Lord, Master Aryon) and enter the only tower you can enter from ground level, a random daedra will spawn in the room upstairs (behind a locked door). If it's not a Saint, kill it, dispose of the body then leave the tower and immediately re-enter. A new random daedra will spawn. Eventually you'll hit apon a Golden Saint (if you are high enough a level for them to spawn naturally). You can do this as many times as you like, though watch you don't make the game too easy. It's a very handy way to get the souls for building your home if you join House Telvanni.