It's said that before Buddhism came to the island the tribes of Sri Lanka worshiped their own deities. That is the Deva, Yaksha, Raaksha, Naga tribes. It's also said that worshiping nature was also done and that Shaivism and Jainism were also present.
For proper sources I think your best course of action would be to ask for some relevant books at the Colombo Public Library or any leading bookshop. They would be able to help you out.