Karl,

If you put a link on your home page that says something like:

Advertise Here!

I am sure that you will find buyers. You just have to give them some proof that your site is worthy of their cash. Show them how many visitors you are getting each day/week/month.

Why not try putting up banner ads.

buysellads.com will help you sell ad space, but they'll also take a decent chunk of your earnings as well. The upside with them is that they handle the actual payments and such, so it's pretty hands free for you.

As Dave said, buysellads.com is a well-know ad network and you can sell your ad spaces there. Also there are other ad networks. For more information check these links, too:
Top 5 Best Alternatives to BuySellAds - TechnoBezz
Partnering With BuySellAds, Email Ad Network LaunchBit Shifts Focus To Lead Generation | TechCrunch
Ad Network Review: BuySellAds - MonetizeMore

If you have any other questions don't hesitate to ask.

Figure out who your website visitors would be helped by if they found a specific company banner on your site.

I have a list of about 2 dozen specific companies that fit well within my niche. I have reached out to some, others have found my site and approached me to advertise.

In the meantime, set up an adsense account and fill your vacant space with ads that can bring in some passive income while you market your site to possible advertisers.

PS: If you don't have one already, put together a media deck. Most company ad agencies or marketing departments will want to know your site statistics and demographics. Having a file ready to email, will be handy so you don't have to scramble when someone requests your site info and rates.

-CG

Putting an 'advertise here' ad/banner on your website.
Tweet to your followers, publish on Facebook, Instagram, linkedin, etc.

Use forums and put a signature offering ad space.

Try buysellads.

Bannersnack(dot)com is another place you can sell ad space.

Mostly I got CAP conversions from top banner of homepages as my little tracking.

Amelia

Use google search operators for easy search reliability.

Example: inurl:marketing allintext:advertise here
allintitle:internet marketing allinanchor: advertise here

Here is a list of google search operators: Google Search Operators - Google Guide

That should save you a lot of time manually searching for the right type of sites. HOWEVER....with that said, there are a few sites out there that will allow advertising that DO NOT have "advertise with us" type of pages or links. These can sometimes be goldmines as they really do not know the true market value of their website.

I typically hire VAs (virtual assistants) to do this sort of work for me.