I think there is a general sentiment here that personally crafted tours, by locals, are a better use of traveler money. But you specifically need a company with the capital to acquire scarce tickets, and arbitrage their value with tourists who, for example, did not plan far enough in advance. Feel free to use Viator.
I have used them for bus tours and private car excursions in various places. In one case, I told the car tour company that I HAD to buy the specific tour I wanted FROM VIATOR instead of from them, because it was not listed on their own website as a buyable item. They replied, "Yeah, we know." I once had to change a date with Viator and the online dialog evenually included "There is no charge for this change", which turned out to be true.