Mixed grass hay should be available at feed and grain stores, horse barns, or farms. If you have several rabbits or don't want to commute too often to purchase it, you may want to buy a whole bale. When you do buy hay, make sure it did not come from the top of the stack where it may be contaminated by animal or bird feces. Simply put a layer of regular newspapers in the bottom of a large kitty litter box and add a nice, thick, soft bed of hay on top of that. Add layers of fresh hay daily between litter box cleanings.
Fresh vegetables are the second most important part of the house rabbit diet.
These foods should be given daily. Rabbits in the wild eat a lot of tough, fibrous plants. Their digestive tract functions best when it has a high level of fiber, which helps to maintain the normal intestinal motility. If your rabbit has never eaten green foods before, then it is best to establish it on hay first. If your rabbit is already eating hay, then there is generally no problem starting right out with fresh green foods no matter what the age of your pet.