Don't know where you can buy a readymade one. However, if you don't mind DIY, try our free software project showmewebcam. It uses a Pi and its HQ sensor and some software glue to make a USB webcam [1]. You'll have a wide selection of affordable lenses [2] and cases [3] that people cook up for their personal use. It's so much fun experimenting with them for different use cases.
Last time I commented here, there have been criticisms about the quality of the lens that the Pi foundation offer. We have discovered many other decent alternative lenses that help remedy the quality and distortion issue of the stock lenses. An example of a good accumulation of knowledge as we have more users and people paying more attention is the commonlands lens guide [4].
The software is very actively developed and we have a pretty supportive developers community. We try our best to have good software engineering practices so we can maintain this project in the long run. The software is designed to be modularized. It is easy to understand, build, and improve upon. I have a lot of fun building it - in fact I just finished a 5 hours coding session to address comments on the Pull Requests that I started earlier. I hope eventually it's not just another pi project for fun, the firmware has the potential to make this solution more powerful than the best webcam that money can buy, just like how openwrt is for routers.
I still have yet to record a decent demo video to demonstrate the power of the Picam but there is just too many things and too little time to get it done. Oh well...
1. https://github.com/showmewebcam/showmewebcam
2. https://github.com/showmewebcam/showmewebcam/wiki/Lenses
3. https://github.com/showmewebcam/showmewebcam/wiki/Cases
4. https://commonlands.com/blogs/camera-engineering/raspi-video...