The best place to start is with Google. Do a search for "racing tipsters" and you will find some to start with. WARNING! Be careful as every tipster is going to guarantee you winners and some will use every trick in the book to convince you they are the best, and this includes downright lies.
You could also try the racing press, namely the Racing Post and Sporting Life.
I personally only use one, which offers a range of services to suit what you want to achieve personally, and this includes a FREE bet service which will murder even the best professional tipsters and they give it away for free. The service is called xWin Investments [www.xwin.co.uk].