You have a few options online and locally, so plan your attack and persevere!
The most consistent source for used, reasonably priced boots is ebay.
As stated earlier, East German officer boots were the basis for Jedi boots. So the best way to get lucky on boots is to stalk the WWII collectibles subcategory on ebay:
Collectibles > Militaria > WW II (1939-45) > Original Period Items > Germany > Personal, Field Gear
Then search for "boots."
But wait, there's more! You can also search for the following not-so-obvious categories:
High, brown leather boots have been trendy for a couple of years now. This is good for two reasons. One, there are more Jedi-appropriate boots being made. Two, when this trend becomes unfashionable, there will be millions of slightly used leather boots for sale. You might want to pop into the Craigslist for your area and keep an eye out for boots. Try a Craigslist watcher or other software if you forget.
Local stables or tack shops
If you live near one or more horses, chances are good that people ride them. And chances are good that those people will sell off their riding bots from time to time. Stop in on a stable and ask where to find riding boots. It can't hurt.