EdMan, I'm sorry, I wouldn't know of the top of my head were such a scroll can be found, but I think a lot of the treasure/scrolls you find in the game are randomly placed and thus are different every time you play through. It is weird however that you haven't encountered a single Spell Trigger Scroll, could it be that you didn't do a lot of side-quests? I suggest you play through the game again (after all, it's so replayable that it shouldn't be a chore to do so) and do any sidequest you might encounter. Also, I seem to remember that the Duergar merchants in the Underdark sold a (minor?) Spell trigger scroll, but I'm not sure.
Oh, BTW, welcome to the board!
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Melusine, Archbabe of the Order of the Holy Flame and the Laughing Hyenas, &
Official Entertainer Elf of the BG2 Bar
Your voice is ambrosia