There's a chap on here – Slowuphill – is a bed retailer, and advised me on a similar topic last month.
See if he can be of further assistance to you.
As it happens, I'm looing for one too. Going to get an IKEA bed frame, but they do Euro-sizing with their mattresses. A quick online review search suggests that IKEA mattresses ain't all that after all, so an independent retailer might be better. (IKEA do, however, provide an online 'matching guide' into which you enter some details about your body shape/size and preferred sleeping conditions, and they suggest which type of mattress would best suit you, if that's a help)
It's a hassle to shop for such an item online, I know.
I agree with the poster who prefers to 'lie before you buy' (I've copyrighted that slogan, by the way).
Good luck Quirrel