You can get various products containing glucosamine for cats from VetUK, which is very reasonably-priced, and if you click through from a rescue, that rescue benefits at no extra charge to you. One of the participating rescues is Paws Inn at
www.pawsinncats.com and there are several others - I rotate them - I'll try and find the link with them all on it.
The dosage depends on the size of the cat. You'll generally be advised to start with a loading dose and then reduce to a maintenance dose. I think Blip's maintenance dose is 125mg, but I'd need to check. She's just under 5kg. The cat products have the dosages on the side, anyway.
Blip is finicky about her food, but eats her Cosequin, which is what we use, happily in her supper.