I dont have a Petco around here, but depending on the company they buy from, you may find it there. Also try your vet, online/dog supply catalogs, or even human calcium....found in the vitamin section .....check the dosage for your dogs size/purpose for it with your vet.
Is there a reason you need calcium supplements? Too much calcium can also be detrimental also. I use a liquid calcium for my pregnant dogs before they whelp to help prevent primary and secondary inertia.
You may also consider adding a little cottage cheese or regular plain yogurt to your dogs diet also. It's a ton easier to get them to eat that than take a pill or liquid!
You need to be extremely careful giving your dog extra calcium. I'd never give my dog calcium unleess it was part of a multivitamin supplement that had phosporus and magnesium in it. Like the Missing LInk. Or unless my vet recommended it.
Too much calcium can actually lead to deficiencies in other minerals, and cause far more harm.
The need to suppliment calcium is very rare. The very best way is to feed raw bones such as cattle femur, so the dogs can gnaw and eat he natrual calcium complete with the other drugs so that its metabolised properly. Then you get bone meal poos which show the dogs digestion of said calcium.
Excess calcium that is administered to puppies is known to cause bone problems such as hip dysplasia, elbow dysplasia, etc etc. Research has been conducted and it has been proven that this is indeed the case. If a dog is fed with a premium food then this would usually contain all the necessary vitamins and minerals so don't be tempted to supply any extra calcium.
I personally DO NOT believe in feeding dogs with a commercial food as I am sceptical and since pet food companies are not policed they can pretty much add or omit what they like and there are plenty of loopholes for them to do as they please.
I feed my dogs and puppies the BARF diet and this is a natural diet and does not require any extra calcium.
Here are a couple of links that go on to explain how detrimental it is to supply a dog with too much calcium.