Jay Trim are quite good or www.wholesaleyarns.co.uk sell it as well in different forms.
What I did though was to find the best price I could and then take that quote into my local independant fabric shop and asked her if she could do it for me slightly cheaper or at that price. I know it might sound cheeky, but I like to buy local and even if the shop owner didn't make as much profit as she normally would by selling it by the metre, it is an instant sale and it isn't stock lying round her shop not making money. I also now buy my 25m rolls of interfacing from her too, so I am doing my bit to keep a local business running and getting supplies cheaper!!