That model uses a uATX power supply which is smaller than a standard power supply. Not many renowned brands make uATX power supplies, and few uATX power supplies are 500W, let alone anything above that. The only maybe-decent one I can find would be
this 400W unit from Antec... which, according to the only answered question on the unit, may not even be from Antec. It has what you need, including a 6-pin cable for the GPU, but if you really want to upgrade, I would advise you get a new case so you can use a full-size PSU, get better cooling, among other benefits.