with hay you can switch it up whenever you want, or whenever you see your chins losing interest in what they are currently eating. you don't have to do a gradual switch like with pellets.
Oxbow brand hay is good quality. i get mine from petsmart (due to crazy shipping rates to get stuff here, especially big/heavy stuff). the bags in petsmart aren't as fresh as what you would get directly from Oxbow (due to the bags sitting on the shelf at petsmart for a while), but it's fresh enough that Rhino likes it and eats it. Oxbow makes an Orchard Grass variety that is a pretty big hit with most chins. they also make timothy.