A friend did this recently but she got her bracket from a large plumbers merchants. If you are near a decent sized town, Yellow Pages might be your best bet. The merchants in question was Falcon at Colchester. If all else fails they might post you one :-)
Falcon Plumbing & Heating Supplies Ltd. No 1 Mason Road Cowdray Centre Cowdray Avenue Colchester Essex CO1 1BX Tel: 01206 767366 Fax: 01206 562624
HTH Dave R
P.S. she found that there wasn't enough head between the top of the shower head and the hot water header tank to get a decent shower. I presume you have checked?