It is a little dificult to tell you for sure what connectors you need with out knowing what size pipe you have. But you best bet for finding connectors would be an irrigation supply store rather than a hard ware store.

I use the irrigation pipe which has a blue stripe down its length, one thing to keep in mind if you are using this pipe is that when it is used in irigation situations it is connected with an external joiner rather than and internal on that you would use to make a hoop. So technically you have to use joiners not desined for that pipe. Luckly there seems to be a connector that fits. I will have a look and try and post the cad number of it when i get home. : )

I buy the connectors from Reese plumbing stores (they have stores all over the place - look in the white pages or on their website) and I just asked for the connectors that go inside the tube with nothing sticking out (this was exactly what I asked the guy and he seemed to know what i meant). They look exactly the same as the ones used in the instructions from hooping.org and cost 55cents. Make sure you get them in the same size as your piping (just ask for that size connector).