There are dozens of online bookmakers with websites. At the time of writing this post, there were 68 betting websites offering sign up offers, but in total there are close to 80. A few bookmakers are mobile app only, but not many. There are four main exchange sites you can use to lay bets, Smarkets, Betfair, Matchbook and Betdaq.
Check out my review of Outplayed or to give matched betting a try head over for the free trial.