I suppose you want to make a sturdy walk-through star balloon arch? Then 1/2" PVC conduit is indeed the best material to use. Alternatively you could try 1/2" aluminium rod, which is easier to work with, but less stable. You can buy or rent a pipe bender to bend the conduit into the desired shape.
Or you buy a pre-made metal frame from Shindigz.com. The star balloon arch pictured below comes with or without balloons for $79 or $99 respectively.
Hope that helps and I'd love to see a picture of your star balloon arch.