This appendix contains a collection of URLs for electronics distributors, sources for mechanical components, and vendors of surplus components of various types. It also includes a brief discussion of buying electronics components and other items from vendors on eBay, with some guidance and caveats.
This is not a comprehensive list, just the companies I am familiar with. This is not an endorsement for any particular company or product line.
Note that many of the sources listed here carry more than just what the list heading might imply, and some sources appear in more than one category to reflect the diversity of things they offer. Major distributors, for example, carry everything from passive components to hand tools and test equipment. Other companies, such as Adafruit and SparkFun, are more focused on the maker and hobbyist markets, but they too carry a wide range of products.