Uh...any number of replacement styluses sold everywhere:
for example play-asia

 

You can pick up spare styli at all game stores I know of.

 

The poor man's replacement would of course be a finger.

Somehow though, I doubt that you could aquire a DS, but $4 for a set of styli is out of your reach.

 

No money, no stylus.

Alternatively, you can use a pen with its cap on (if you have a screen protector it should be fine). If not, those cotton thingies to clean your ears should be good, too.

 

You are in the US, right? $4 can't be too far out of the question if you are. Take back some bottles or something. Mow a lawn. Rob an old lady. $4 is pocket change these days. It'll only buy you three candy bars.

 

Q-tips work. Put one in a pen.