Buying new mattres for US crib in the UK

I have a US-sized crib but would like to purchase a new mattress for it here in the UK (where the cribs are different sizes than in the US).  Does anyone have any experience buying a mattress in the UK to fit a crib from the US?  It seems like it may be a possibility to get one custom-made, but I wasn't sure how expensive that would be??  Or, has anyone found a UK-sized mattress that works in a US crib?  Thanks!

I've seen a lot of custom crib/cot mattress options -- most of which are the organic, coir-filled variety. 

Most of the sites were like this:  http://www.thelittlegreensheep.co.uk/The-Little-Green-Sheep/Custom-Mattress.aspx

You plug in the specs and they give you a price.  They were fairly expensive IIRC -- at least £200.00.  Although if you've already got the crib, it might well be worth it.

One thing I noticed was that UK cot mattresses are much thinner than US ones -- they're uniformly 10cm thick.  I am currently using Pottery Barn US crib bedding on a UK cot-bed mattress and it fits decently, if not perfectly.