There are dozens of tour companies which offer the Sani Pass tours (often charging ridiculous amounts IMO), however all these companies only provide transportation to Underberg. The actual Sani Pass Tours are operated by 3-4 different companies based in Underberg which charge very similar (and reasonable) amounts (~600ZAR per person).

My tour was operated by Major Adventures: excellent experience, friendly staff at the office and very knowledgable tour guide / driver.