FWIW, I'd try Freecycle, Craigslist and eBay. You could even get a new pair of Nashbar panniers for $50, that will be far more efficient and attach more securely to a rack that what most people can cobble together on their own.

I also wouldn't sweat the whole freeganism thing. Feeling any "inner turmoil" over a mere $600 is not worth it, especially since a well-made / well-equipped bike can last you 20 years or more. And just by using a bike for your travels and transportation, you're head and shoulders above most people in terms of carbon footprint, social responsibility and so forth.