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.
Don't want to create the frame yourself? This 5ft 7in star frame sells for under $50 at Amazon.
Hope that helps and I'd love to see a picture of your star balloon arch.
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