I recommend trying any place but walmart, they are cheap bikes that often don't last, and they are one size fits all. Bikes are not one size fits all. And especially at over 200lbs, you want good brakes. I recommend mechanical disc brakes at the least, and recommend hydraulic discs.

I have had rim brakes on a cheap bike fail on me while going down hill. Not a pleasant experience.

If you are competent at mechanics and feel you could finish assembling a bike, I recommend checking bikes direct. I got my son a Motobecane 529 with hydraulic discs from them. Right now they are $399. They have the Motbecane 400ht with mechanical disc brakes for $299.