You can keep hay in her cage ALL the time. It's a false economy to not feed her hay. That's what keeps their teeth ground down and their digestive tracts moving normally, thereby saving you very expensive dental and vet bills.
By far the most expensive way, per pound, to buy hay is from a local pet store. It will cost you three to five times as much as ordering online, and hundreds of times more than buying from a local farmer. However, where you are, there's no good locally grown hay. You might find some that's shipped in to a feed store, or you can order online.
Guinea pigs need a long strand grass hay -- timothy, meadow, blue, orchard, brome. Good places to order include KMS Hayloft, American Pet Diner, Small Pet Select, and Sweet Meadow Farm. Each of them offers various sales/coupons/etc, so you can shop around and see what you'd have to pay.
The critical thing to remember is that you have to look at the price per pound. One of them may have a box for $50, and another one for $40, but the $50 box may have 15 more pounds of hay in it.
Something to think about... If you can't afford to feed her hay, you can't afford the guinea pig. Hay is the cheapest thing she'll ever eat. She needs pellets and fresh vegetables, and when she gets sick, an exotic vet. Dog and cat vets aren't equipped to treat guinea pigs, and some are downright dangerous.
