Where can I get some preload spacers?
Finding spacer material really only applies if you're buying used springs or OEM EX500 ones. If you buy aftermarket springs, they'll come with a spacer that you can cut to fit.
If you have springs but no spacer, the usual recommendations are to go to the hardware store and get either some schedule 40 PVC pipe or thin-walled steel or aluminum tubing. PVC is the most commonly used material. If you take the fork spring washer out of the top of the fork, you can take it to the store and see what size diameter you need.
In all cases, a washer on top of the spring, between the spring and spacer, is necessary. The spacers sit against the fork caps, so as long as there's a clean/smooth edge on the spacer there is no need to have a washer on the top end.
Cutting the spacers to length
When fitting new spacers, make the spacer length so that the stock spring + spacer length is maintained. Repeat: Make your new springs plus spacer the same length as the stock spring plus spacer.
Cut the spacer as close to straight across as you can. Using one of the factory edges, so you only have to cut one end, is a good idea. Remove all burrs, making the ends as smooth as you can get them. If you're cutting metal, you may want to find a machine shop that can do a square cut for you. For PVC, you can use a mitre saw set for 0 degrees on both axes. A hacksaw, used carefully, will usually do an ok job, but a pipe cutter will give a perfect, square cut every time. Works on plastic or metal.
For small changes (a few mm) you can always use large steel washers on either end of the existing spacer.
How about putting in a longer spacer?
One of the 'cheap-out' Band Aids that has been done by some people in the past is putting long spacers in their forks in lieu of stiffer springs. This is positively not recommended, and here's why: By adding more preload (the spacer), you make the bike stiffer/harsher for everyday commuting/riding, and it'll still be all mushy and wallowy when you push hard. That's kinda the opposite of what most riders really want. So, you're really much better off putting in new, correct-rate springs than using the Band-Aid formula.
Good, Cheap, Fast... pick any 2.