eBay and CraigsList.
Actually, I like CraigsList because if you can find, it's typically local and you can drive over straightaway to pick it up.
For commercial use, your best bet is going to be a commercial grinder. While I use a Baratza Maestro at home and know it will grind great coffee, if you need to push volume, the Baratza will kill you.
Look for used Bunn G1, G2 or G3 grinders. The difference between the three really is the hopper size. Also, the Grindmaster 875 will work. Every once in a while you'll find these grinders in your price range. Don't be afraid to email them and ask if they'll sell it for $150. I recently bought a $3500 piece of lab equipment that was selling on eBay Buy It Now for $500 simply because I emailed the guy and asked if he would take it.
The Ditting and Mahlkonig grinders are excellent and if you happen to find one for $150 but it straight away! In fact, if there are two, buy them both and I'll give you $300 for the one and you'll get yours for free!