Where is the cheapest place to rent snorkel gear in St John for a few days?

purchase at home and pack in your carry on.

We stayed in Coral Bay in May. We took our own snorkels and rented fins at Crabbys.

Go to any number of Scuba Diving shops and rent for the week. They will usually help you get the best fit. Fit is the most important aspect to snorkel or dive gear. Maybe even ask them for tips on proper use and best spots.
Low Key Watersports and Cruz Bay Watersports to name a couple.

We've rented from Crabby's several times and have been satisfied with the equipment and service. If you are planning to return the equipment in the morning, check in advance to make sure the shop is going to be open when you want to drop them off. His morning hours aren't consistent.