You can do a search, as there have several threads on bikes at Hilton Head.
You can rent from any number of companies on the island, and they will even deliver and pick up your bikes.
I however prefer to rent right at the resort from Disney. It may cost a little more, but it's still a good deal, and really convenient. Last time I was there it was $50 for two bikes for the length of stay, and $10 for each additional bike. They were new, good quality bikes (Mongoose brand). And if you had any problems (or even got the chain full of sand from riding on the beach), they would gladly fix it or swap it with another bike.