Listed by Cross Common Nursery - rather expensive
http://www.crosscommonnursery.co.uk/citrus-and-tropical-fruits/orange-trees/citrus-orange-sanguinelli/c-1451/
Also may be available from The Citrus Centre in Sussex.
Note that there isn't really any such thing as a 'dwarf' blood orange. If grown in-ground it's the rootstock that controls the size. If in a pot - presumably the case in the UK - then the pot size is the major controlling factor.