Floor Style sells used and refurbished sanders. Or they used to:
http://www.floorstyle.com/
Also, check with the local hardwood tool distributors. They sometimes have repaired and returned sanders to sell. There's also Craig's List:
http://sfbay.craigslist.org/
Just click on YOUR city and start shopping.

You can also try your distributors. I purchased a clarke classic-8 from one of my distributors. gave $1600 for it and it had a brand new drum on it it's used but it works good and I got it before they started to refurbish it. Totally pleased with it except for the fact that it's a pain to haul up stairs!!

Good luck. Reliable used machines are hard to come by. Stay away from anyone trying to sell you a Clark "Apollo"8. I bought one brand new about 10 years ago thinking it was the bomb. It was a bomb alright! unless you like your floor looking as wavy as the Atlantic ocean. Been using a good ole Hummel 8" ever since.

Good luck and congrats on the decision to strike out on your own.

Another note on the Apollo 8. I almost bought one til I heard they were discontinuing parts for them in 07. I'm afraid my classic 8 will be phased out next.

Oh yes, the built in obsolecence factor. Hopefully, Floorstyle and others will continue to carry parts for those older Lincolns, Americans, Clarkes, Holt-Pullmans, Silverlines and others.