Most resorts in Mentawai include cultural presentations in their packages, and the highlight of the presentation is dancing with "Stone Age" shamans.
For a more authentic feel of the dance, you may want to hire a boat or go on group tours to Pulau Siberut, 120 km off the coast of Sumatra. Siberut is the largest of the Mentawai Islands. Ferries leave four times a week from Padang and takes two-and-half hours to get to Bukittingi, where you can get accommodations, supplies and guides. From Bukittingi, you'll have to hire another boat to Siberut and ask your guide to negotiate with a shaman for a dance. The shaman would not charge any fee, but would expect that you would give some tip after the presentation.
Take note that boat trips from Padang to Bukittingi have erratic schedules, so double-check first before going there. It's also more expensive and time-consuming to get your guide as you arrive. The trip from Padang to Siberut should only cost $100-150, all in, including guides and porters. May to September is the best time to visit to avoid the rainy days and the mud.