Not reading any more of this thread as it's setting off my diet paranoia, so apologies if I'm repeating PPs.
Peak Supps and Bulk do bovine protein powder (dried beef, basically). I use it because it provides everything beef does - amino acids, etc - except the fat. It's an easy way to increase dietary protein. Since I'm only using it to top up a normal diet, a kg lasts a long time so it's economical.
It comes in sweet flavours, but I use the neutral one to mix in with food or make soup bases, etc. Make sure you don't order the bovine collagen if you want the complete protein product, they're listed together.
For arthritis, Glucosamine & Chondroitin has longer research behind it and does help a lot of achey people, including me. It comes as a combined capsule/tablet, which you can buy almost anywhere.
I don't know what to think about collagen - I understand it doesn't seem to make sense metabolically but people have been using gelatine and bone broth for centuries to support healthy hair, skin and connective tissue. Maybe it's all been bullshit for that long, maybe not?!