As is generally recommended, my rabbits are getting pellets and hay. Although websites usually mention Timothy hay as the most preferred, that's pretty pricey (Tractor Supply sells a 50-pound bale for $23.99 as of this writing). Bermuda and alfalfa hay are more available. The link above suggests that rabbits don't like Bermuda grass as much, but the nutrition profile of Bermuda and Timothy are essentially identical (see Table 1 of the linked PDF). One note is that alfalfa hay has more calcium and is usually contraindicated for rabbits in most articles I've read (e.g., this one). This is not a problem; alfalfa costs more. On Craigslist, alfalfa bales are usually about twice the price of Bermuda, which are the only two readily-available options other than the prepackaged and expen$ive route from a Tractor Supply or similar store.
When I got rabbits, the only hay I had on hand was some Bermuda bales from the previous winter--I park the chicken tractors in late November and don't move them again until March. I tossed some in the cages and the rabbits immediately began nibbling at it. They may prefer Timothy to Bermuda, but they seem to go for Bermuda with enthusiasm nonetheless.
I'm not sure exactly how much they actually eat vs. lose through the wire bottoms of their cages. Some people have built little hay racks in their cages, but I just dump a bunch on the floor of the cage. Sometimes there's still some left the next day; sometimes not. Some of the rabbits seem to get into playing with it:
In the foreground is the red New Zeland buck; the TAMUK buck is in the background. The one in front often burrows into the hay as soon as I put it in his cage--but he eats it, too. Tossing his hay around though is bound to make more fall through. Alas. He has gained weight very well, and rapidly, so he's getting enough to eat one way or another.