See if your town has a needle exchange program, like a lot of U.S. cities do. This is an AIDS-prevention program that supplies clean, new darts and pins to drug users. You turn in your used darts to them (which is a great way to get rid of them) and get new ones, either free or for a very small fee.
If you don't have any to turn in, no problem. They just might charge you a dollar or something. Their maximum charge here in California seems to be about one dollar for ten new darts. Nice price!
AND they give you a card, so if police find the darts on you, you can show you got them legitimately.