Band of Starpower

The extra mana matters most in the short window where you are still on first tier magic weapons and running dry halfway through a fight. Roll it into the Mana Regeneration Band as soon as you can, since that version keeps the mana pool and adds regeneration on top. Once you have the upgrade, the base band has no reason to occupy a slot.

Crimson worlds have no Shadow Orbs, so the normal route is closed and you have two workarounds. Crafting it in a Graveyard, using a Mana Crystal plus a Panic Necklace, is the faster path if you happen to have both pieces already. Otherwise you plant an artificial Corruption from Corrupt Seeds, which the Dryad only sells while she is standing in a Hardmode Graveyard, then fish crates out of the patch you made. Both are a lot of effort for a mana accessory, so this is really a completion goal rather than a power spike.