The size should be stamped on your wheel rims near the stem holes. It should say something like DID JAPAN 1.85X18 meaning 1.85" wide. This is also known as a WM1 wheel. From what I can remember the 500 had 18" WM1 on both ends with 40 spokes. Your best bet - despite the cost - is Buchanan's. They know what you will need and they will help you and back up their product if there's a problem. If you're in the UK, there are equally good suppliers there. Ebay stuff might fit and might not, and there's no guarantee you can expect to be honoured. The thing with alloy rims is that most have a "depressed centre" that means stock length spokes are too long. Buchanan's will sell you a rim and spokes that will fit with your hub or replace them with the right ones if not.
Measure the clearance from the rim to the forks and brakes in front, the swingarm/chain/everything in the way in back. Figure out what tire will fit in there - the widths of tire sizes can be found somewhere. The front tire has to fit through the narrowest place - usually between the fender mounting bolts. You can cheat this a bit witha deflated tire but that's a pain.
Now you know the tire size you want, pick the rim that's best for it. This chart
http://www.hagon-shocks.co.uk/WheelTab.htm shows the rims that work with the tire sizes. Each tire works with 3 or 4 rim widths: pick the second largest. Now see if the rim type you want is available in that size. Usually it isn't... but 40 spoke rims (ie Harley compatible) are more common that my 36 spoke ones... sigh. Anyway, pick the rim and width you like and have them drilled for your rims - if the seller doesn't know what that means buy from Buchanan's... they do. Get spokes the right length and size plus the right size nipples (see previous sentence).
There are some shops (ie Buchanan's again, Hagon, etc.) that will build your wheel for you. Just strip and clean your hub and send it: then you get a nice new wheel back assembled by an expert.
