Where can I find weather data on ambient outside illumination?
One direction to look in might be the various GIS systems that exist to forecast the amount of light that will reach photovoltaic (solar panel) installations. I don't know about the US, but I would be very surprised if there isn't one - try googling for "PV GIS" or similar.

Now, that will give you insolation (illuminance) in W/m2. It may be further broken down into direct and indirect insolation (that's direct sunlight and diffuse light from the sky or clouds), or it may be a single global figure.

Since you're after the information in lux rather than W/m2, you'll need to convert it. This will necessarily be an approximate process, but so long as you don't need very high accuracy it should be quite doable. Lux is at base a measurement of power density, just like W/m2, but it's modified by a frequency-dependent function that represents the sensitivity of human vision. So you'll need to assume a spectrum for the sunlight, and then apply the conversion curve (which is an international standard, and hopefully available on the internet).