You can rent them for a decent price at Snorkel Bob's in Hawaii. At least they were decent prices back in 2004. You can rent from any Snorkel Bob's location on any island and return it to any location on any island. I think we paid $10 to have the gear for a week. It wasn't top of the line stuff, but it was good enough for a couple of beginners. You can pay more to rent higher quality stuff. They also help you to properly fit the stuff and show you how to use it.
Their website is slow for me but didn't have info on rentals, but I found another site where users are talking about renting from Snorkel bob's. Pricing seems to have gone up but its still a decent deal.
Click here
For my wife and I, renting was the way to go because we were only going to use the equipment while we were in Hawaii and we didn't plan on going anywhere else where there is snorkelling for quite some time. Its worked out well because nearly 5 years later we don't have equipment we never used again and there is no trip in site where snorkelling is involved. If you plan on snorkelling on a yearly basis then you may want to buy gear.