How feasible is it to buy sunscreen (at least SPF 30 but preferably 50) on Caye Caulker? Any particular place that stocks it?
If anyone who's similarly pale can help me out I'd really appreciate it!

How feasible is it to buy sunscreen (at least SPF 30 but preferably 50) on Caye Caulker? Any particular place that stocks it?
If anyone who's similarly pale can help me out I'd really appreciate it!
There are at least two large Chinese markets and a bunch of smaller ones. You can get anything you need and more.

Greggo is quite correct. The Chinese-run businesses he mentions are large general stores. During your first wander up or down the main drag you'll run into them. They keep very long hours, and are open seven days a week.
They've got all kinds of food items, hardware, general merchandise, bicycle rentals, and more or less anything a tourist would commonly need. However, although they will have sunscreen with a proper SPF level to be effective, it may not be the same brand you are accustomed to. Also, the prices are likely to be higher than in North America or in Western Europe. If you're concerned, then bring what you need from home.