I was looking for a way to grab Darkvision, but couldn't find anything thus far.
Yeah, I went through this with a character not long ago. It is kind of a pain finding something appropriate that isn't too expensive.
A wand (or eternal wand) of the spell could work. Or if you could add it to your spells known somehow, that could work, too (I forget which ones the Tome assassin can pick from). Or a custom magic item. Or hell, you could have it made permanent. I'm not sure how we're going to account for Permanency with our wealth system, though. Hmm...
How about we say that one of Mystra's divine proxies (who was also your old supervisor) used Permanency to make Darkvision permanent on you as a job perk? I've given out special things for a few other characters (custom PrC, races, Sphere, etc), so I don't think this is out of line.