Online Advertising Agencies for Niche Marketers

My business, targetmeister.com, sells target stands for shooting. I was exclusively using Google AdWords for advertising and that has been effective, but I need more traffic/sales. Today, I signed up for Microsoft adCenter and Yahoo Search Marketing too. However, I feel I'll need to make a more targeted effort in order to get in front of more potential customers and increase sales revenues.

Can anyone recommend any other effective online advertising agencies for niche marketers?

You might use some more Organic Search positioning. For instance, I just googled for "target stands" and I didn't see your site in the first 4 pages (I didn't continue through all of them). Even though your site looks great and professional it has very little text content which doesn´t help Google to index your site properly.
Additionaly, you may try google analytics to get some data about traffic sources and organic visits (those who come from search engines).
Hope it helps

Have you tried to do the Ads yourself, and offering the ads to companies that sell products and services related to the content in your website. Yes it implies more work in your part, but you get to keep 100% of the ads' revenue and you also have full control of what ads are shown in your website.

I know that the website stackoverflow.com is doing something similar to what I just described, I believe they tried Adwords but it didn't work well for them an so they decided to do their own ads and sell them directly to the interested companies.

Hope this helps.