It depends what breed it is I suppose.If it is TB try the Curragh Bloodstock Agency and if it is a sporthorses then try your local vet.He may be able to advise.
We have about 16 well bred sporthorses at the moment and not one of them is insured.
lol, its only for little shetties! a 5year old and a 1year old. neither have been insured before and never needed them to be but thought i'd take a look and see! they don't do anything and aren't worth alot but didn't know whether it was worth it