How to reach potential customers with limited working capital/ideas for cost effective marketing?

[Jake]

Mass media is a rather blunt instrument, self serve content ad networks like Google's Adwords allow you to place ads on participating sites of your choice, you can even target individual pages with articles particularly relevant to prospects. Google's other form of ads are sponsored search results, you can only pay for clicks by those searching for "Staffing Service" etc. These ad systems also have geographical targeting so you only pay for visitors in your region. Prices are established by competitive bid, I suspect search ads for you might cost over $1 a click.

Facebook paid ad demographics allows you to only target employees of select large companies.

Here's a specialist in cheap local TV spots
http://www.cheap-tv-spots.com/