There are a couple runes that help with direct mana sustain, such as Manaflow Band in the Sorcery tree and Presence of Mind in the Precision tree (I wouldn't consider things like Biscuit Delivery to be a mana sustain option).
Occasionally, you might start the game with Corrupting Potion, which gives okay mana sustain. It's a good starting item if you want to opt a more harass and early aggression focused lane approach to start your game (it has health regen, mana regen, and some burns). I have a personal bias for Corrupting Potion start myself on some of the champs I play.
Other than that, there are 3 typical mana starting items you can rush if you need mana. If you are someone as special as Ryze who literally needs mana (by having a mana kit that requires you to spam) and actually use Archeangle's Staff or some oddballs who like to use Muramune/Muramana, prepare to stack the Tears of the Goddess. If you're going to trade more often against your lane opponent, get Catalyst. If you just wanna sit back and play the poke and farm game, get Lost Chapter. Or if you think you really starve on mana like Ryze typically is, get at least 2 of them or even all 3; but only if you actually build something out of them otherwise it's a waste of gold, which I should't need to tell you.