As dogs age, they can begin to experience stiffness and pain in their joints. There are many canine "joint supplements" on the market today, but unfortunately most of them are animal-derived. Luckily, there are alternatives.
Glucosamine is one of the most widely used joint supplements for dogs. It is usually made from shellfish, but in recent years, companies have found a way to make grain-derived glucosamine.
Vegancats.com carries Prescription 2000 Glucosamine HCL, in an easy to use powder form.
Deva Glucosamine with MSM & CMO comes in tablets. MSM helps produce collagen, and CMO reduces inflammation.
Both the Deva and Prescription 2000 glucosamine formulas are marketed for humans, so you just have to figure out the dosage for your dog. The average dog dosage according to a vet is 250 mg per 10lbs given twice daily.
UPDATE: I have found a third vegan glucosamine supplement, made specifically for dogs. ActiPet Vegan HA Supplement contains plant sourced glucosamine, MSM, Hyaluronic acid, Vit C, organic alfalfa leaf, and manganese. It is available for purchase from Vegan Essentials here.
Vitamin C is suggested as possibly helpful for arthritis. It is found naturally in many fruits and vegetables, and also in supplement form.
Omega fatty acids might also be beneficial. Good vegan sources for omega fatty acids are flax seed oil, hemp seed oil, canola oil, olive oil, and soybeans, among others.
Acupuncture, massage, swimming, mild exercise, and physical therapy can be very helpful to arthritic dogs, as can keeping them from becoming overweight.
Please remember to check with your veterinarian before putting your dog on any supplements or exercise programs.