Can a wizard, through levels, races, paragon paths, epic destinies, or feats, ever gain more at-will spells?
Be a Human
That'll get you a third power from the choices you had at level 1. There's no other way that I know of to "purchase" more abilities from that list.
But don't stop there. You can always...
Poach from other classes
Check out the Half-Elf Polymath paragon path.
Broad Dilettante (16th level): Choose a 1st-level at-will attack power from any class that is different from yours. You gain that power
Or take a Multiclass feat like Binding Initiate.
Benefit: You gain training in one skill from the binder warlock’s class skills list. You also gain proficiency with rods and wands. Choose a binder pact boon. You gain the at-will attack power and the utility power it grants at 1st level as encounter powers.
There are plenty of ways to get other at-will powers. Some feats/paragon paths/epic destinies/items just straight up grant you at-will powers, though they aren't always attack powers if that's what you're looking for. Creating and maintaining a comprehensive list of those would be next to impossible. You're better off looking for something specific you want your character to do rather than just amassing as many at-wills as possible.