I would highly recommend this site. I purchased a model from them four years ago and it's still running strong today. Very heavy duty, gym quality treadmills and most of the machines are rated for heavier people as noted in each of the descriptions. I had one failure in the upright locking mechanism (my son actually broke it) and had a no hassle free repair performed within 3 days with the included warranty.
http://www.fitnessblowout.com/equipment/treadmill