I recently bough a used guitar, I went to adjust the pickups yesterday and found that the screw for adjusting the neck pickup on the treble side was completely stripped, so I have 2 questions:

1. how do I get the screw out?

2. where would be my best bet to go for a replacement?
1. Needle nose pliers and a shit load of patience is probably all that will work on those screws.

There is an old trick used for Nintendo consoles to mod them as the used non standard screws.
Get the plastic casing from one of those cheapo pens. Melt on end of it and while it is still hot shove push it into the screw, making an indent of the screw. then wait for the pen to harden, then stick it back in and you will have something perfectly molded, and use that to unscrew. I'll try find you a link to how to do it.

2. The hardware shop or a guitar shop.
whichever is closer. Hardware shop will have what you need. Possibly for cheaper that the guitar shop, seeing as the guitar shop would probs want to put the screws in for you as well (which obviously isnt needed) and they may want to charge you for that.