They look through normal ads like the rest of us. And i know London got a couple from cobham manor in Kent. One of their horses 'Deano'was a £11,000 dressage horse!!
Yes they source them as other people would, charities, ads etc, they were once interested in one of mine.... but I decided to sell privately - for no particular reason.
They bought a full ID youngster off a neighbour. The horse had been advertised in a freeads paper. The youngster made the grade and is regularly seen out and about on patrol.