Sun Electronics in Miami, but I don't think they do installs. Not positive about that. Many of their offerings have minimum purchase requirements, but some don't. If you're even minimally handy it's pretty easy to install yourself.
As far as adding to existing system, it's most important to match panel voltages if you have an MPPT controller. If you have PWM it isn't as critical. I have one odd panel out of 4 and the system still works great (the three matching panels came from Sun). But if you know your existing panels' ratings you might be able to find something that matches.