Betfair has an interface where you can see "depth of market", more or less. Their software was originally designed as a kind of reconstructed "level 2" stock screen, I think.
(Edited to add: sorry, posted at the same time as Jack and without having seen his post).
My son has been doing a lot of sports betting this last year on numerous accounts I have had to set up for him.
He's mainly concentrated on European Football - including accumulators bets and as found various specialist sites dealing with La Liga as well as sports arbitrage deals that seem to be easy with all the offers being made by so many new bookies etc wanting to attract new business
His best accumulator netted him over £300 of a total £5 bet - but according to what he's told me some guys have been getting really silly wins of punts - ie like £2000 of a £1 accumulator and one lucky punter had 7 wins on various footballs teams one saturday - all with over 3 goals scored in each match and was reportedly a £40 k winner of just a few pounds stake.
Sounds to good to be true if you do all your homework etc with the potential for great RR ratios.