Here are a couple of options for Vitamin C supplementation that doesn't come from citrus:

Rose Hips - contains 10 to 100 times more vitamin C than other foods
Red Raspberry Blend - comes in liquid form in a vegetable glycerin base and can be taken straight or made into a tea
Barley Juice Powder - 7 times the vitamin C content of oranges
Hawthorn Berries - helps increase intracellular levels of vitamin C and good for the heart and circulation

Of course, you can also get Vitamin C Ascorbates, a powdered form of vitamin C that is non-acidic to the stomach. The vitamin C is a mixture of calcium ascorbate, magnesium ascorbate and potassium ascorbate and there's no added sugar.