The ones Police use are costly because those are backed with an expert who can testify in court showing how they work and that the vids weren't altered. Most are overpriced and have less video resolution than you'd want and many aren't sold to the public.
The so-called 'spy cams' built into ink pens, sunglasses, etc you can get vary from rotten to decent. Keychain cams and even our old friend Mobius can be used discretely if you have a hand free. Most folks use their phone for that nowadays. SpyTec has a range of cams for this and you can find them on Amazon and Ebay too. I just let my car's side cams do the job of covering people approaching my car doors which is a free bonus to their traffic protection function
There are police style consumer body cams entering the market. In some cases they are manufactured and marketed by the same companies that make professional police cams. Some offer expensive evidence management systems for chain of custody, etc. while others are used more like typical action cameras. One even has a dashboard mount available. Admittedly, I don't know much about them but I've seen a number of different body cams offered for sale.