There is a veritable multitude of companies making spas today. They are usually manufactured in acrylic for a deck or a specially constructed area inside the house. Often spas are sold a lot like cars — there is lot of upselling and steering buyers towards a particular brand. The Pool and Spa Living Magazine has the most information online about spa selection, including a buyer’s guide. There are two models that seem to be favorably reviewed quite often: the Marquis Coastal Series Reward (priced around $6000) and the mid-range Emerald 599 (priced around $3900). See this report by Consumer Search for more information.
See also this Spa Buyers Q & A from the Spa Specialist, which has answers to frequently asked questions about buying a spa.