Banner Ads

You have multiple options in case you want to try banner advertising. The common options available are

1) Going directly to the publisher
This is probably the best option because you can (theoretically) get the best price and the best location for your banner in this way. Finding sites in your niche might be a problem - but there are good options if your willing to search. I would say the easiest way to proceed here is to search for the best forums in your niche and see if they accept advertising.
E.g. - An SEO course vendor buying banners on WarriorForum and BlackHatWorld to reach potential buyers for his course.

2) Going to a premium marketplace
This consists of marketplaces like Buyads (from Isocket) - https://www.buyads.com/ and BuySellAds - Buy Ads | BuySellAds. These marketplaces have a selection of medium to small sites in multiple niches and you can book your campaign on desired sites using these marketplaces.
The benefits for the media buyer is a more transparent deal (you know the exact location and placement of your banner) and guaranteed placement (if you buy 1,000,000 impressions using these platforms on SiteA, these 1,000,000 impressions are guaranteed to you - these is no way that another advertiser can come and take these impressions by paying a higher CPM)
Please try and search for sites in your niche on these sites.

3) Going through ad networks
Networks like the Google Display Network (GDN), Advertising.com and others aggregate display inventory over multiple sites. These networks bring scale and reach for advertisers (and are often cheaper than the first 2 options). They also allow multiple targeting options - demographic, contextual etc. But the flip side of networks is less transparency and non guaranteed placements (you could book 1,000,000 impressions for this week, but the network has the rights to sell those 1,000,000 impressions to a higher payer).

4) Using an RTB platform
Real Time Bidding platforms like Perfect Audience and SiteScout allow advertisers to buy across multiple ad networks and use cookie data (and email ids in some cases) to target these users across multiple networks. These platforms are a relatively new entrant into the display ecosystem and can be really useful in the hands of someone who knows what they are doing!

Check out Bonadza. We offer exactly what you're looking for. There are over 40.000 websites to choose from, and we work with standard sized IAB banners.

Check this out: Convert2Media Resources

Thank you all for the advice. I decided to test out an ad using BuySellAds. I'm just wondering if it is better to direct traffic to a minimal landing page as opposed to a full website. I'm even considering direct linking.

But anyway, I like BuySellAds for finding direct advertisers. Now, I just need a couple of good networks that do PPC display advertising. Anyone can recommend a good one (or two) with low minimum deposit? I looked at Bing but it seems not to have an option to display banners for advertising.

+1 for BuySellAds.com. Recommend them highly.