Alibaba.

Most other shops will be expensive, occasionally you can find deals on anchor optics clearance (though that info is years old, and to have to know what you're buying!).

You're almost always better off buying the cameras and sensors together, unless you have specific needs. Sadly, this isn't something where you can save money by doing it diy. Supply chains are good and overall demand is too high for deals outside the normal channels, I think.

Would love to be proven wrong, but I used to pay around with optics regularly and it was always more expensive than I liked. Cutting open photographic lenses was a common way to get fun glass for cheap... So getting cheaper glass is tough.