I think you're going to have a mission to find a hub that's made specifically for a Fiat 500, since there's nobody out there using a 500 in motorsport.
If there was a Fiat 500 rally series or something, then some firm would have made a hub, since quite a lot of people would want one.. but the 500 is only really used as a road car, so it's just a few individuals who want to change the wheel/remove their air-bag and are not bothered that it will fail its next MOT....
As above, it is an MOT violation to remove an air-bag. Cars don't *have* to have one, it's true.... but that means cars that didn't ever have one fitted. Any car after about 1995 is 99% likely to have an airbag fitted.
The other thing to think about, apart from having to bribe your MOT tester every year and what might happen if you have a disagreement with an articulated lorry, and the airbag warning light blinking at you since it can't detect the airbag any more.. is when you sell the beast in a year or two... who is going to buy it?
I'd look at sticking nicer (road) wheels on it, to get it looking saucy. Leave the airbag alone.
Ralf S.