I think that Allen & Page are one of the lowest calcium content pellets - I remember looking in to it a while back for my buns, it was hard to find but I did get it.
Supreme have also launched a Urinary Tract problems special pellet (Vet Care Plus Urinary Formula or something similar) and it is supposed to have the lowest calcium content but I think its around the same as A&P Its very expensive though, but has been formulated for buns with these kinds of problems. I feed some of these pellets in with A&P to a couple of my buns.
ETA: I will try and find the links for you but it may not be til later on as I have just popped on quickly in the middle of other jobs.
Some calcium contents on this page, but not A&P or the one I have just mentioned: http://www.rabbitrehome.org.uk/care/rabbitfood.asp
According to my box the Supreme Vet Care Urinary Formula has 0.5% calcium.
This link says that Allen & Page has a calcium content of 0.59% http://forums.rabbitrehome.org.uk/sh...ium+allen+page