Most BC players play on Kongregate, and it's a fairly large site. It's easy to play right in-site, making it an ideal place to advertise.
I'm not sure what the budget for BC is, but Kongregate might be willing to work out a deal since BC has made them money on ad revenue and kreds, and will continue to do so as it grows. One of those full-page background ads ran for a while would be a great way to get a lot of new players into the game, if a decent graphic designer made it.
Heck, if it really comes down to it you could try and run a gem sale or something where all profits go towards the ad, I'm sure plenty of players would love to chip in towards growing the game.