The manufacturer/supplier here in Aus doesn't do air domes. Must be a UK only thing, in fact the guy who makes them here (so I've been told) swears the idea/design was stolen by the UK manufacturer.
I wonder if just running some 4mm hose around the bottom of the pot near the drain holes and plugging it into an air pump would give similar results? Or if anyone has experimented with different kinds of aeration methods for these things. In any case my current plant seems pretty happy without them. Something to try on my next grow I guess.
Although on different forums people have such massively differing views on the domes, the consensus on one forum seems to be don't bother and on another they're great. Just a matter of suck it and see for yourself I suppose.