You can sometimes purchase replacement parts from the manufacturer of older cookware. Auction sites such as eBay.com have used cookware parts, and online retailers such as eReplacementParts.com carry a wide variety of replacement parts for vintage cookware.
Replacement parts for major brands of cookware are easier to find. First, check the manufacturer's website for replacement parts. If you own obscure brands or cookware from manufacturers that are out of business, you may need to look at enthusiast forums related to the brand. Craigslist.org and local estate sales sometimes have older cookware that you can use for parts. In some cases, cookware parts might be interchangeable between brands.