$20 a month is quite a small budget to rank high in the SERPS but still you can use that money to get quite a few high PR links on monthly basis.
I'd recommend looking for blog roll links and intext links in particular for the time being.
Other than that, just use your creativity and do a google search you'll find thousands of auto-approve blog lists to use, you can do it manually if you can't afford a tool, and after all you don't need 50K link blasts believe me. Look for quality instead of quantity.