Hi ,how long is your trip? Neps are everywhere in Borneo - common lowland are easily found along the roadside ( gracilis, mirabilis,green and spotted ampullaria, raff to some extent).
To see lowland,intermediate and highland . In Sarawak- Bako national park, Gunung Murud, Mulu National Park. Sabah - Mesilau Nature Park and Kinabalu National Park( cool temperate weather 12°C - 17I° C ). If time is too short skip Kinabalu. Mesilau offer the best chance to see Rajah, burbidgea, edwardsiana,etc ( if u are lucky-ask the guard-NOT GUIDE )
. From Mesilau the trek up to 9000ft-10,000ft will connect the Kinabalu main trail to the summit,you will see Villosa here-the GUIDE will tell you,but tell him/her your main interest.
Guide in Sarawak- there are many travel agent.check with their web site and tell them your purpose and get a guide who know neps.Usually they are trained to serve the purpose- to see bird or wild life or neps or native culture or all
Guide are also available within the park itself.
Robert