There are a couple of very cheap ways to go. First you need to know if they are legal where you live. If not, you will need to find something else to do.
Get a golf tee and take it to your local hardware store. They should have some 1/2 inch white plastic water pipe with an inside diameter the same size as a standard golf tee. You only need ~ 3 foot piece. Use golf tees as darts.
At the same place, get a piece of electrical conduit pipe about four feet long. By darts online (they will need to be sanded down on the back end to fit the conduit), or make your own.
For each of them design or copy a mouthpiece from the web for yours.
You will also need a piece of paracord or other cord tied to a slender rag to clean the bore every now and then as spit builds up from steady shooting.
From one or both of them you will have a good idea of what you really need or want. Those I know who own Cold Steel blowguns really like them. But they started out cheap before committing.
I have a four foot electrical conduit blowgun a friend made for me and it serves me well for what i want from it.