Sometimes replacement parts are not available anywhere. A problem came up for me recently, and I was able to create the part using AutoDesk Inventor and 3-d printing technology.
The Porter Cable 557 type 1 biscuit joiner uses plastic parts for the key fence hinges. These parts are prone to breakage and the supply of spare parts is exhausted. Any web search for "part 151" will turn up only part suppliers who report that the part is no longer available, and forum posts from people who are trying to find one.
For the lack of a $1 part, a $200 tool is rendered unusable.
Here's how I learned all this: Not too long ago, I picked up a biscuit joiner cheap on eBay. The ad said that there was one broken plastic part. Not realizing that it was a critical part, I bought the joiner. After a month of fruitless searching for a replacement part, I realized I would have to make my own or junk the tool.
Step 1: The tool
At this point, it was either scrap the tool, or come up with my own pivot piece.
Step 2: Measuring the part
The first step was to get a pair of calipers and take all the measurements I could.