search google and this website:

Labuk Bay Proboscis Monkey Sanctuary
Sabah, Malaysia

personal experience...

I have been lucky enough to visit here a number of times with the now defunct Orion Expeditions.

background

It is only a small piece of land, with the main habitat being mangroves.

the star of the show...the proboscis monkeys

This is a great place for photographing Proboscis Monkeys close up. Proboscis monkeys come into feed regularly at the various feeding stations, which have large viewing platforms.  It is also good for watching their behaviours, particularly the dominant males. You can also see the females, (the ones with the cute upturned noses) and their differently coloured young.

a male Proboscis Monkey, feeding station, Borneo. (image by Damon Ramsey, www.ecosystem-guides.com)male Proboscis Monkey have big noses

There are other visitors, too, including other primates, such as macaques and silver leaf monkeys.

portrait of a monkey

 I have seen hornbills and flying paradise tree snakes.

access...

It is located just outside of Sandakan, and day tours can visit the site.

Updates
Throughout the virus I am working in Australia on and off as local borders close, mostly as in the Kimberley with Coral Expeditions (May-September). If you can't go travelling until everything settles down, then until then, here I am doing online guided walks for Noble Caledonia and online lectures for Silversea.