ebay is your best bet. you need the main gun only, the barrel is useless . You will replace it with the PVC pipe instead.
The main change is that you need a 1 1/2'' to 1 1/4 coupling. The 1/2" goes in the tip of the" build a blaster" (it will fit small so you have to dremel it inside to make it fit. Don't dremel too wide because you need it to fit tight in order to hold the barrel (you can use some E600 also to seal off). Then, you attach the 1 1/4" barrel to the coupling and bingo!!!! say hello to my little friend!
Going from this>>>
to this>>>>>>