I am interested in getting banner advertisements for my website and just wanted to know if there are any reputable advertising agencies that would sell banner advertisement for my website for a commission?
It's not agencies you're looking for, it's ad networks, or site representation firms. Ad agencies are the ones buying the ads, not selling them.
Just do a Google search for ad networks ... a billion should come up, everything from the ones that put crappy ads on your site for pennies, up to the more reputable ones. A lot of them are niche, too ... if you have no experience selling advertising online, then check out a whole bunch to become familiar with the industry. Just don't sign any exclusive contracts!