urgent brood mare help needed please

Try going to all the big horse magazines online, they usually have forums which they could join to post a help message and wanted ads are sometimes free of charge.
Also try World Horse Welfare, BHS, SSPCA, National Foaling Bank (they sell proper mares milk replacement and I think colostrum which is vital) and any other studs within reach. Presumably their vets will have contacted other veterinary practices locally, but they could spread the net a bit wider.
Any mare who will 'mother' the foal will help to give it encouragement, even if they still have to hand feed, and there's often an old brood mare floating round at studs who will accept any foal given a little encouragement, at least then it would grow up knowing it's a horse as hand reared foals can have severe personality problems as they get older.
Best of luck...this happened to a friend of mine once, she bottled hers for nearly a month before they found a mare who had just lost a foal. The little welshie was slightly startled to be presented with a 'new' foal who was already as big as her (they skinned hers and wrapped my friends girl in the skin and rubbed her with dirty straw from the foaling box) but raised her happily. My pal slept for a week solid to catch up from two hourly feeds.

Hope you find a mother for the little one.
I was really interested to read Its Hilly's comments about personality problems with hand reared foals. Many many moons ago when I was a teenager I worked at a livery stables. When I started they had just lost a mare (twisted gut) who had a foal. I can't remember how old he was or whether he was weaned. Anyway he was always difficult to handle we stable girls would much rather handle the gentle giants than the unpredictable foal. In time he was sent to a racing stables and did race a few times but became more and more unpredictable and dangerous. He came home for a while but sadly was put down as despite careful handling and nurturing was dangerous. I think he was about 7.

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