oddschecker is a great one, Healy - definitely poke around as there are plenty of other good resources as well. Sometimes one place (say PinnacleSports) will have action on something you'd think
oddschecker should have but doesn't - and vice versa. It's good to have plenty of cushion to fall back on.
And yeah some approximation of the mean or average from different sites (using your own judgment) is the best way to go, I've found.