Timothy hay is what most people feed their pigs, but if you have problems with grass allergies, orchard grass tends to upset allergies less than timothy.
You can find hay at the pet store, but most of it is sold in very small quantities and the quality may not be great because it usually sits on a shelf for a long time. Another option that a lot of us choose to do is buy hay in bulk from an online supplier. The quality tends to be MUCH better because it comes directly from the farm, and you can get enough hay to last months. Obviously, the up-front cost of buying a large bulk of hay is more than buying a little baggie of it, but per ounce, bulk hay costs a fraction of the hay sold in pet stores, so you save a lot of money long term. Some of the favorite bulk suppliers of timothy hay for small pets that are commonly used by members of this forum are KleenMama's Hayloft, Oxbow, American Pet Diner, Small Pet Select, and Sweet Meadow Farms. The cheapest and best option for hay if it's available to you is to purchase a partial bale from a farm or a horse barn.
Most adult pigs need around 25 mg vitamin C daily, but they don't really need a vitamin C supplement unless they are sick or elderly. Providing 1 cup of a variety of fresh veggies per pig daily is the most important thing to ensure they are getting the vitamin C and nutrition that they need. Just be sure to include veggies that are high in vitamin C and lower in calcium, such as bell pepper.