The Similans are surrounded by open ocean some 50 km due west of Khao Lak beach and are reached only by tour company operated speedboats. While many of these disembark from Thap Lamu pier, which is the official Similans pier near national park headquarters in Khao Lak, many tour companies have their own piers in different locations, though none are too far from Thap Lamu.
Many tour operator offices are located around Thap Lamu pier and throughout the Khao Lak area, and it's also possible to book through resorts, guesthouses, and travel companies (they will receive a commission for this service but the price remains the same for travellers either way as the tour companies typically pocket the commission for themselves when you book direct through them).
Day tours cost between 1,800B on the low end up to 3,500, and it sometimes pays to shop around and fork out a little extra for a better experience. These rates should include lunch and snorkelling equipment. Also note that it can be worthwhile to choose the tour company you want first, and then ask for rates from a few different travel companies as some offer cheaper prices than others (and presumably take less of a commission). With this being the case, travel companies often quote cheaper rates than the actual tour companies. We went with Fantastic Similan Travel and were very pleased with their service from start to finish.
Also note that while many travel companies on Phuket offer day trips to the Similans, you'll still first be shuttled to Khao Lak unless going on a diving tour, and Phuket companies are known to charge steeper rates than those in Khao Lak.