It's tricky with older cars. Depending on the brand you're after you might have to hunt a while. When I had a 2nd gen, I got some of my parts from UltraRev. They might still stock some of the older stuff. Hard to say.
Beyond that, it's worthwhile to figure out what you want and then start googling. I've heard of numerous stories of people finding old parts this way that have long been discontinued. Retailers stock parts and often have old ones laying around their warehouses. Can be hard to find.
Don't discount eBay as well. Sure, there's lots of new junk on their, but it's also the first place a lot of people go to offload things that have been collecting dust in their garage. It's a great place to locate hard to find parts.
I'll mention this thread: viewtopic.php?f=25&t=916
There you'll find a list of the manufacturers that were making parts for Preludes in the 90s. Some of these parts of been discontinued, but as I said, it's still possible to find them.