Try oodle.com and look to see what website the ad was posted from they can also lead you to good websites. Go to auctions( make sure the horse is not sick or lame or have arthritis) Horse prices have really plunged. Because of the economy a lot of people are unable to afford feeding extra horses and will sale them cheap. Bigger horses as in draft horses? Clydesdale, Fresians, Jutland, Noriker,Murakozi, Poitevin, Shire, Sokolsky, Percheron, Normandy Cob, Lithuanian hd(heavy draft), Italian hd, Irish d, Dutch hd, Finnish hd, comtois, Brabant, Ardennes, Auxois, Boulonnias, breton, Russian dh, and Rhineland draft.
Seriously, remember the adage 'you get what you pay for'. Cheap horses are cheap for a reason. You must be in or around horse country since you're talking horses, so you can also visit stables and boarding barns for leads, place 'horse wanted' notices on community bulletin boards, and check local newspapers - horses for sale. Just a thought.