Wel...
[Without any Cash Shop or Leeching gear]
If you don't have access to neither or don't want to use them, you farm/buy Strawberry or another SP regeneration fruit (Mora Mandarin, Lemons, etc.). They're lighter and generally cheaper. One cool tip if you can tank her, find a Bacsojin (either natural spawn or ET) and farm Iris from the Green Maidens she summons XD Best SP Regen item. In the worst of cases, just stop, sit down and wait.
[+Leeching Gear]
You can always buy the leeching gear, be it Cash Shop or not, from other players. Evil Dragon Armor is the most versatile by far. Rideword Hat/Vanguard Helm are the runner ups and much cheaper, even at +9. Depending on your classes it's the one you should get. Thanatos Weapons are so good too once you're 120. A Hunter Fly + Thanatos Weapon + Succ/Inc Pet and I rarely ever need to use Straws. Once I added the EDA, SP has never become an issue when leveling. Like, I haven't used Straws since I got it if I'm wearing it.
[+ SP Regen]
Stuff like Arc Angeling I've never used because I'm almost always over 50% and when not, I'm using EDA, so, yeah :v No need. Don't have any word of expertise here.
Really, SP sustain is only a problem if you're leveling as fast as possible. If you take it slow, you can always return to town, rest at inn or just plain sit down. When I'm leveling with pals, we always enjoy the sit down phase at lower levels when Leeching is not as efficient (because lower dmg output).