I can't think of a single "Ready to Ride" ebike that has a rider weight rating of more than about 250 pounds max . My best advice would be that you consider having someone either build, or help you build, an ebike conversion (i.e. kit bike) based on a heavy duty Worksman bicycle. I also recommend a power system of no less than 500 watts @ 36 volts with a battery capacity of no less than 12AH (you will still need/want to peddle as much as you can stand). Such a kit might cost you anywhere from 1200 to 1700 USD (not including labor). Note: most electric bicycles and motorcycles, like their four wheeled counterparts, are going to cost about as much, or more, than an internal combustion engine driven vehicle of somewhat similar performance. Electrics just haven't come far enough along yet to where they're competitively priced with their fossil fueled counterparts.
Meanwhile, here's a few ebike kit and ebike retailers that might be able to help you:
1. Electric Bike Solutions (http://gocarlite.com
). You'll want to talk to Doug (he has a shop near the Bay Area).
Btw, Doug is a super guy to deal with (great customer service too).
2. Ebikessf. This S.F. CA based shop is operated by a very helpful guy who is well known in the ebike community (he has some pretty high powered/high end kits and bikes).
Also keep in mind that there are more than a few of us in here that can help you out if you decide to build your own ebike.
Btw, welcome to the forum.