Yes, the supplement I use is for dogs only. You can find it at any pet store, really. The one I use is called Liquid-Vet.
It's not necessary, but it is good for dogs that are very active or do high-impact activities. Helps promote joint strength and hopefully decreases chances of joint injury. I started using it after I noticed Squirt had a bit of a limp in his right hind. Luckily it was nothing serious but it did make me more concerned about future injuries, so I decided to start the glucosamine.