Look for a skateboarding shop like:
Val Surf
22864 Ventura Blvd.
Woodland Hills, CA 91364
PH: 818-225-8177
They may not have what you're looking for, but most likely they can point you in the right direction.
Over here most shops like that also sell spray paint cans and different kinds of caps.
Most likely you want one of these: Gold Dots, German Outlines 2, German Outlines (world's most popular and precise cap), NY Thins.
Take the can with you, so you'll be sure it'll fit (1 cap will probably cost $0.50 or less).
**EDIT** If you can get your hands on an injection needle you could make one yourself. (I used to do that when I was young, I've yet to see anyone beat those extremely thin lines (Less than 1/4 Inch)).
**EDIT 2** You might need to practice a bit when you're going to use a needle, as you need to move pretty fast.