I'm not sure where you live
but if you can find an outfit that sells directly to customer, that might be a good option.
I only have experience with innerspring types so I cannot comment on water beds, air beds, foam etc.
For my money, we have an outfit in Pitt called Original Mattress factory. Started by a chap that used to work for Sealy, they make their units and sell them in their own stores. They sell real 2 sided mattresses and you can customize them to a point like having foam bolsters placed in the outer coils to provide a firmer edge. I've had my set for 11 years now and I sleep very well on it (queen size)
If your locale has such an option, I recommend it.
I would not buy a brand name mattress. Especially the so-called "never flip". That is nothing more than a total bunch of marketing BS. THere is no such thing as "technology" that makes these things special. All they do is leave out the padding, etc on one side (thus saving costs) and promoting it to the lazy ass who refuses to flip and rotate. And true to form, the typical American moron fell for it (but you still pay full retail for essentially half a bed!)
FWIW, nearly every outfit buys the innerspring coils from Legget & Platt.