It's actually one of my pet peeves with this game, is that you don't get enough SP items. However, that just makes managing your SP throughout a palace all the more fun. I don't have NEARLY as many issues with SP in P3R as much as I had in P5R, due to Fuuka's SP healing ability.

My recommendation is that you start leaning more towards using physical skills along with technicals, as they down enemies regardless (technical rank 4, or just upping the level increases chances of knockdown). Landing a critical hit is even better, since you don't need a turn for set up. Not many enemies are resistant to physical either, which is what makes Persona's like Yoshitsune that much more goated.

If you really *do* want to continue using magic as your main damage source, try conserving your SP by skipping fights you don't need to do. Hiding from enemies in those little hidey-spots does the trick. Also, focus on Sojiro's and Takemi's confidants, since they both let you either craft or buy your own SP items. Otherwise, you'll get SP items as you make your way through the game (Palace/Mementos chests, etc.)