TredStep make amazing boots! http://www.tredstep.com/prod_boots_giot.html
While I don't have their paddock boots, actually have the tall boots, I find them to be the best quality out of every other brand I tried. If the paddock boots are anything like the tall boots, the quality of both the stitching and the zippers will leave you very pleased with your purchase.
Another good brand is Ariat... I had their paddock boots and they held up OK for a year and a half. Whatever you do, stick to the major brands like Ariat, Mountain Horse, TredStep, etc. My first pair of paddock boots were some random brand that I found just to save a little bit of money and they barely lasted 6 months. Sometimes spending the extra $$$ is well worth it in the long run. :)
I would suggest eBay
btw! I have bought brand new boots with shipping included for anywhere from 25% to 50% cheaper than retail pricing. If you wait, sometimes you'll find an amazing deal!