I source replacement forks from my local co-op, sometimes from friends in the local bike community, and most often, eBay. I've also bought touring forks from Soma, and once from Action Bicycle for a Peugeot UO-8 restoration.

You can usually get a decent made-in-Taiwan lugged steel road fork on eBay for $50-80. It starts getting pricey when you want one with a specific brand engraving (like Eddy Merckx, Colnago, etc.). One-inch threaded touring forks (with canti studs and extra rack mount points) are also harder to find. Predictably, French-threaded forks are often cheaper and there's a nice supply on eBay.