Depends how big the site is. Less than $20k/annual net you're not going to have much luck with a broker.
Beyond that, it is always worth speaking to a broker (just make sure you do your research first!). If you want a non-biased recommendation from a broker (me!) have a chat with QuietLightBrokerage as recommended above.
Centurica (also mentioned) are a due diligence firm. They don't sell businesses. Flippa is okay if you have a site <$20k in value.