Write/re-hash an informative guide around anything, which requires the service you want to sell. The service itself doesn't have to be in the focus. Insert your links casually in it, spread the guide whereever you can (forums, social media sites etc.). Make sure, it's actually a good guide and not a sales pitch. Costs nothing but your time.
For instance one example i did back then, i wrote a CPA approval guide. Most of the time you'll need a website and hosting, if you want to work with CPA networks, so i inserted my hosting affiliate url and obviously CPA referral links too. I was selling this on forums, Fiverr or even giving it away for free. So i made money with the guide itself and through the affiliate/referral urls.