I saw these in a circus last weekend and would like to get a set. No idea what they are "officially" called, but they looked like straw hats, and had somewhat of a boomerang effect to them.

Performer had 3 of them and would toss them out (they flew rather slow - graceful) and they'd return to him. With the set of them, he was able to juggle them by tossing them outward.

Anyone know what these hats are? More importantly - a supplier where I can get a set?

Yeah you can do it with Frisbees and cards if you are good too. It is in the technique more than the hat. Get a light wide brim hat and go out in the open and experiment. Start throwing it up. Then start angling it out until you find the best angle. Then get good at it. :)

and pray for no gusts of wind during a performance.

With a Frisbee I normally start around 45 degrees and then vary it a little (usually up a little higher) for the best results.