I started looking at them a while ago with a view to getting one for William, who has damaged discs in the lumbosacral area. You can buy them in Argos but I wanted to actually look at some to see how high they are, so waited until I went to Crufts. Big disappointment, as I only saw a couple and didn't think much of them.
This article
http://dogcare.dailypuppy.com/pick-height-elevated-dog-feeder-3329.html advises how to measure the correct height. I'm not sure that it's necessary to be that precise so at the moment I'm trying makeshift things of various heights, like folded dog towels stacked up, a low step stool or a small cardboard box to see if William appears more comfortable at a particular height.
The other problem I might have with a commercially bought one is that I won't use plastic bowls for the dogs. I prefer stainless steel or ceramic, so that might limit my choice.