There is also a random chance that anyone who sells scrolls will have some, but no Merchants sell them by default (just in leveled lists in their containers.)
It's possible one of the LGNPC mods changes that. There's a quest (I'm sure it's LGNPC) for the wife of the enchanter in Aldruhn (who's been locked in her room for "losing" some special arrows) to obtain 3 of those scrolls, and I know I found them in a magic seller very quickly. I don't remember having sold any to them at that point, but I could be wrong.
I'd sure like to change those levelled scrolls for something more useful tho. Why open a lock with a scroll when you could be practising and improving your security skill with good lockpicks instead? Same with keys, I have hardly ever needed those except at lowest levels. You don't have to be a grandmaster thief to open most high-level locks even with journeyman picks, and those and better can be found all over the place, you don't have to buy them. My character has a barrel stashed full of master/grandmaster picks and probes that are almost never used, as I get enough practise from wasting cheap apprentice tools on lower-level locks to be able to open
anything with journeyman tools.
Plus, the one time I did try to use a lock-splitter scroll, on the chests in the Dren Plantation which only have a level 90 lock, it fizzled out without result for no apparent reason. As far as I was aware, scrolls were guaranteed casts. Whether that particular chest was scripted to resist a magical unlock I don't know, but I've never wasted my time (and septims) trying to open locks with scrolls since.