Here is a general guideline by weight for OTC medications including glucosamine. It's handy to bookmark.
http://www.walkervalleyvet.com/otc-meds.htm
Each product will have the recommended dosage on the bottle, though. Many start with a loading dose, then taper down to a maintenance dose after 4-6 weeks.
Be sure and get a good brand like Glycoflex or Cosequin instead of a cheaper brand. The less expensive brands don't have enough glucosamine and you'll have to give twice as much.
Any of the online pet supplies places have it - KV Vet, Revival, Fosters and Smith, etc.