I ran across some soft-tip blowgun darts (.40 cal) on Ebay, that let you shoot them at electronic dart boards. Wow! Are they ever fun. One caution though: Unless you plan on shooting from 20 or 30 yards, you'll want to take your blowpipe apart and shoot with just a 2' or 3' section. Otherwise, the dart hits so hard it will bounce back out and fly who knows where. I get a lot of double takes when I take them into a pub and shoot at the dart boards.
I got 100 of these for $4.00+ shipping.