The rogue must be able to see the target well enough to pick out a vital spot and must be able to reach such a spot. A rogue cannot sneak attack while striking a creature with total concealment.
It seems they don't have the same problem.
Ignore partial concealment
Feat -> permanent
The Shadow Strike is a simple solution available to anyone at the cost of a feat.
"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)."
Doesn't grant vision, but can allows stabbing anyway and overcomes not only low-light but also fog, rain, blur effects, displacing effects, etc. Note, this won't work for complete or magical darkness, or any form of total-concealment.
See in the dark
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.
Characters with low-light vision can see outdoors on a moonlit night as well as they can during the day.
Dwarves' Senses Racial Trait darkvision grants "perfect" sight up to 60 ft (still black and white?).
Darkvision is the extraordinary ability to see with no light source at all, out to a range specified for the creature.
Class: Dip -> permanent
- 2 lvl in Umbral Weaver (bard) gets low-light vision.
- 1 lvl in Wild Stalker (ranger) gets you low light vision.
- 2 lvl in Shadowdancer (prestige class with many prerequisites) gets you darkvision, and improves its range if you already have it.
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+SL=30. Also if your intelligence is less than 10+SL=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
-Mask of the Mantis costs 6,000 for 3h of darkvision / day. To be exact it has 5 charges per day, 30 min each that can grant 60 ft darkvision or a choice of some other things like +5 to Perception. You can activate multiple effects at the same time.
-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.
Let there be light
Alternatively, don't stab in the dark.
Light -> until dispelled
Have someone cast Continual Light on your weapon (60 gp);
Flank or feint or deny dex bonus another way.
Note: Sneaking might still work as long as you keep the dagger hidden until you attack; certainly don't go sneaking around like a glowing x-mas tree...