Ah, cables! You're in for a treat!
As Lucy mentioned, the size of the cable needle doesn't matter, just as long as it's smaller in diameter than your knitting needles.
DPNs are double-pointed needles. You can pick buy them in packs of 4 or 5. They do make great cable needles (as well as the other household objects mentioned.
). The best part is, when you're done with your cabled poncho, you can use them to make matching socks!