Hello Archon,
You get more and more of them raising city levels.
You get some each time you hit your higher available level of Persians positions, provided you have engaged the relevant quest.
You get some from every tournament.
Actually, I always have a hundredth of spare ones, meaning I get them faster than I can use them. the reason is mostly because each study takes at least 5 to 6 hours to complete.
So maybe I can suggest an idea to help you using your scrolls to a similar rate than you get them :
Higher skills levels take more time to study. So once you've unlocked a new level, rather than always going on investing in new (thus lower) levels, you could go on using them in skills you have already unlocked (and that you wish to upgrade anyway, of course), to raise them to higher levels.
This way, your studies will take more time, time that you would have found too long later when you would be wanting to upgrade them in the future, and that would then be done already when you'll get there.
And since you currently have to wait until you get your next scrolls, it would use this otherwise wasted time efficiently. :)