A number of laces (some on the internet;do a search) sell 2-piece 4 to 6 foot blowguns in .40 (the most common caliber). They'll also sell a number of darts of different design/uses.
"Breath of Death" (the book by Micahel Jainch mentioned above) has a wealth of info on making your own darts anda rrows from easily obtainable materials. Even those unforunate enough to live in Ca. or Ma. can make their own, perfectly adequate, blowguns.
I make hunting darts from bamboo skewers, paper cones for fletching and x-acto blades for tips. My tube is .40, but I'm looking hard at the .63 Big Bore.
My first suggestion would be to do a web search, as there are tons of sites out there that cover every aspect of blowgunning. Then decide if you want to make or buy your 'gun and darts, then go to it. It's a fun hobby, but requires a LOT of skill for hunting. Stalking takes on a whole new meaning with a blowgun.