Greyhounds always play '2nd fiddle' so the task becomes difficult.
My friend tells me that the Racing Papers advertise regularly (thoroughbreds) bookmakers and what they offer. ie Sportsman, etc etc which come out I think on a Friday morning. (tomorrow !)
Of course, this does not answer your question, but it might give you some 'leads' which you could follow up.
Then again if you 'google' say - bookmakers greyhounds
you might get some leads.
BTW it would be a valuable tool to any punters if you are successful
in answering this question.
Sorry, that's all I can offer.