We used, and I would recommend 1/2" metal conduit, 10' long. They were about $1 a piece at the hardware store.
Now you can bend the conduit by hand, you can make your own bender, or you can buy one. We purchased one from Johnny's Seeds.
We tried making one, but the hoops came out fairly wobbly and wonky. This bender gave us perfect arches. The arches don't have to be perfect, but they certainly look a lot nicer, and if your garden is visible from the street, you neighbors might appreciate it.
I have 6 garden beds, each are about 4'x8'. I put three hoops per bed. That means the hoops are bout 4' apart.
So, I needed 18 pieces of conduit.
Once you make the hoops, there are a few different ways you can install them.
I have raised beds, and I installed the hoops on the outside of the bed. It gives a little more room to the plants, and the hoops are not pushing the bed apart. It makes mowing the lawn a tad tricky, but i don't mind.
My favorite method was to push the hoops about 6' into the ground, and I secured them with a pipe clamp.
You can also just put them in the ground. They stay up fine, if you don't have kids swinging on them, or if you don't expect several feet of snow.
The most secure way, although time consuming, it to buy some 3/4' conduit, cut it into 2' lengths, hammer them into the ground, so only a few inches are remaining, and insert the hoops.