I've been searching for a suppler of xray film in the UK and have come up with these guys... www.veterinaryxrays.co.uk
Or more specifically...
They don't have an online store, but if you contact them via email they'll get back to you with prices / ordering info. When I last looked into this they were offering 100 sheet boxes of 18x24cm for £47.50 (ex VAT, I think...), although that was back in April, so their prices may have changed.
Hope that helps!