You can try your local Craigslist site or ebay. Of course a treadmill is quite heavy so if you can't pick it up yourself you have to figure in the cost of shipping.
You can find an inexpensive used treadmill in Houston at www.preownedfitness.com or at http://houston.craigslist.org/spo/.
You can find used treadmills for sale on the auction website Ebay.
Ebay has lots of used treadmills for auction or buy it now.
There is an old axiom that you get what you pay for. A treadmill with a warranty is unlikely to come cheap, and a cheap treadmill is unlikely to come with a warranty. Your best chance would be to visit a secondhand sporting goods store and then try to negotiate for a warranty. Your only option is to find a treadmill that still has it's orignal warranty left on it. Otherwise there are no companies that specialize in used treadmill warranties.
A good quality used treadmill can last a lifetime with proper care.
People do sell used treadmills for good prices at auctions. You should go to Ebay and do a search for "used treadmill". You can often find treadmills for as low as $100.
Check out www.craigslist.org in that area for used treadmills people may be selling for cheap. You can also try www.overstock.com for great deals.
You may be able to find one on craigslist.org or oodle.com, they are both free sites where people can resell their used personal belongings.
You can get a used treadmill from eBay. There are also a number of used treadmill on Amazon. There are exercise machines for decent prices, depending on what you're looking for in both places.
The best places to find things that are cheap and used are amazon.com and also eBay.com. These two websites, especially eBay, specialized in used items sold at a cheap price.
The most economical place to find cheap gym equipment is on eBay or craigslist or any other venue to purchase used equipment.
You can find a cheap price of a Honeywell generator on the internet at Amazon. This site can be used to compare prices before you decide what one you want to purchase.