Check out thrift stores and garage/estate/yard sales.

I just got finished looking over your thread about your bike. I'd say a) you did get a decent bike for your needs and current range, b) it looks like you're either on a very tight budget or just have a problem with spending money in general (or justifying the bike related expenses in particular- I can relate, as my wife frowns every time I spend a dime).

Your lowest cost option will be a rack like that Topeak, your current backpack, and some bungee cords. Or a milk crate ziptied to the rack.