Cosequin is the best IMO. A lot of the glucosamine products out there don't have enough to be beneficial. For a dog Lady's size (7.5 lbs.). my vet said she need 300 mg a day. Most of the less expensive "off" brands don't have nearly enough.
I get it at Entirely Pets or KV Vet Supply.
Cosequin Regular Strength (90 capsules) KV Vet Supply / KV HealthLinks - Pet, equine & livestock supplies / Quality nutrition for you!