For some time now, Cindy has been using a different Vitamin C supplement for guinea pigs needing an extra nutritional boost due to illness or prior neglect and malnourishment. The supplement comes not from the pet store, but from the baby supply aisle.
Natra-Bio Children's Vitamin C comes in a sweet, citrus-flavored syrup. Unlike vitamin supplements available in pet (supply) stores, this is not a multivitamin -- it's all Vitamin C. (Guinea pigs don't need multivitamin supplements.) Cindy says the rescue pigs she's given it to have all lapped it up. Look for it online for the best price (and make sure you check the expiration date when it arrives). Any local drugstore, vitamin shop, or natural grocery should be able to special-order it for you if their distributor carries Natra-Bio.
Another option is actively recommended and prescribed by South Wilton Vet. Maxi Baby-C Liquid from Country Life is a vegetarian/kosher supplement that comes in an all-natural cherry flavor. Guinea pigs that don't like oranges, or orange-flavored treats, will likely respond more favorably to this supplement. Look for it online to get the best price (and check the expiration date when it arrives). Like the Natra-Bio supplement, if your local store doesn't already have it on their shelves, they should be able to order it for you.
In both cases, you'll have to have your own syringes in order to administer these supplements to your guinea pig(s); supplements are administered orally. For an adult guinea pig (average weight: 2.5 pounds), give 1.0ml daily. You can administer 0.5ml twice daily if your guinea pig doesn't have the patience to sit for 1.0ml in one dose.
(NOTE: Just as your own doctor needs to be aware of what vitamin supplements you're taking regularly, your veterinarian needs to know what your guinea pig is being given.)
If you find other options on the market, be mindful of the ingredients. Watch for milk, lactose, or any ingredients derived from milk (like sodium caseinate); guinea pigs are herbivores (no milk, no meat). And, again, always check expiration dates.