To get the best prices you'd need accounts at all the sites.
And you need to use Betfair.
What do you mean by 'taking sports betting seriously'? You surely don't mean make a living from it, do you?
yes its a bit of trouble opening so many different accounts, but you will always get the best price available + a great bonus .
some great offers about,
http://www.online-betting-guide.co.uk/free.php