Race -> permanent
Elves, Half-elves and Gnomes have the Senses Racial Trait Low-Light Vision allowing them to see twice as far as humans in the dark.
Feat -> permanent
The Shadow Strike feat works. It's nice to be able to see in the dark, for all kinds of other reasons, but this is a super-available one feat solution for the sneak attacking issue for any darkness focused build who can't see in dim light and other shadows. Note just like low-light vision this won't work for complete or magical darkness.
"You accurately strike even those you cannot clearly see [...] You can deal precision damage, such as sneak attack damage, against targets with concealment (but not total concealment)."
Class: Dip - permanent
2 lvl in Umbral Weaver (bard) gets low-light vision.
1 lvl in Wizard makes the process described above easier, as it saves you the trouble of having to UMD, since then the spell would be on your class list.
1 lvl in Wild Stalker (ranger) gets you low light vision.
2 lvl in Shadowdancer (note many prerequisites) gets you darkvision. However, if you get the spell done first, the class might further enhance the DV you already have.
Item: Money/Use -> 3h / 90 gp
You can avoid paying for it with a feat, if you're willing to pay with cash instead. As KRyan points out you can get darkvision with a wand (4,500 gp/50 uses * 3 h/use).
If you plan on using darkness a lot (more than 150 h), you can alternatively go through a really complicated process to make it permanent for just a little bit more.
Spell: Money(~7k)+Plot -> until dispelled (by an even more powerful wizard)
To do this go search for a powerful wizard (10 or higher CL), to make a scroll of Permanency (level of the spell (5) × the creator's caster level (10) × 25 gp + 5000 gp to the cost for the diamond dust = 6250 GP), then get his apprentice to cast Darkvision on you (Caster level (2) × spell level (2) × 10 gp = 40 gp), and then UMD to cast the scroll yourself. The DC is 20+CL=30. Also if your intelligence is less than 10+CL=20 you'll have to make a separate check DC 35. With an int of at least 16 you could have the apprentice cast Fox's Cunning (+4 int) at you for another 40 gp. Either way Eagle's Splendor (+4 cha) will should be worth it for the +2 bonus to UMD and get a custom UMD masterwork tool for +2 (a bargain for 50 gp). Maybe the wizard is willing to "aid " you by explaining how finishing the spell (which is really all you're doing) would normally work. Maybe you can get ahold of some other temporary boosts. Maybe you can roleplay some of the preparation (that perhaps took days), and a cool ritual in which you read the scroll... However it is possible, with significant effort and at the cost of the equivalent of about 1.5 wands of Darkvision - if you don't mess up and destroy the scroll...
Item: Money -> permanent until lost or destroyed
-Goggles of Night cost 12,000gp and grant 60 ft darkvision.
-Darksight Goggles cost 20,000gp and grant 120-ft darkvision, also when underground: +4 competence bonus to perception and tracking (survival).
If dispelled they're just deactivated for 1d4 rounds, so very permanent. Not super complicated. Ideal if you can afford it and your game has magic items easily accessible.