Not sure about orthopedic mattresses, but bought a mirrorform 6' mattress from mattress.ie. They were up to 40% cheaper than other stores or other (Irish) websites, so I guess they must be pretty cheap... They do free delivery and are reliable.
I'll second the recommendation for mattress.ie, got the same mattress I'd seen priced for €560 for €389. With free delivery and great service. Definitely worth a look. Do a search, I've seen posts on mattresses on here before.
A good quality mattress is very important, after all you spend a third of your life there. You really need to lie on a mattress for a few minutes to decide if its for you. Buying any furniture over the internet is not recommended unless you can identify the exact product and have already tested it in a shop. If you find the exact same model you like cheaper on line then certainly go for it, otherwise buy in a shop.