This would depend on the game in question. Generally speaking, you should aim for the Classified Ads section of one of the forums. for example, if the game is Animé themed, you'd go to the BYOND Animé Classified Ads. You can find a direct link to each of the main banner guilds under the 'Community' menu.
When posting on one of these forums, please ensure that you did read the requirements and advice carefully. It would be very beneficial if you would add more information as well. You want others to do something for you, so provide them with the information they need and show them that you're willing to put up with the tasks ahead by showcasing the effort you put into creating the advert.