This is a work in progress page about the NanoVNA (Vector Network Analyzer) project that produces these things for about $50. The “version 1” of these devices is available on eBay, or Amazon.
There is a second generation model in the works. The RTL-SDR folks have a great discussion of the changes between the first and second generation boxes. The newer boxes are currently running about $100, and are available on AliExpress at this link.
The definitive source of information is located on a Groups.IO page. Anyone can join, at this link.