The most famous place in Borneo for getting the chance to see Turtles is Turtle Island located off the east coast of Sabah. The beaches of Turtle Island are visited every night of the year by two endangered turtle species, the green turtle and the hawksbill.
Turtle Island (Pulau Selingan) is one of three islands in Turtle Island Marine Park dedicated to green and hawksbill turtle conservation, and the only island which permits visitors to view turtles nesting and hatchlings being released. The turtle hatchery and the turtle conservation programme on Turtle Island (managed by Sabah Parks) is the oldest in the world. To visit Turtle Island you need to book an overnight stay, typically as part of a 2D1N tour.
Nearby islands on the east coast of Sabah, such as Lankayan Island and Libaran Island, also offer a similar experience but on a smaller scale.
If you are a scuba diver you also have the opportunity to see Turtles during dives at Sipadan and on islands located close to Sipadan, like Pulau Mataking. With Sipadan being a turtle nesting site you're pretty much guaranteed to see Turtles (and several of them) on every dive. Turtles are also found along the coral reefs fringing the islands in Tunku Abdul Rahman Marine Park located in front of Kota Kinabalu offering both snorkelers and divers the chance to see them (if you're lucky).