I can't tell you where to buy them but you may query your state highway dept.
Panels about 2' x 3' and a battery pack right under the panel. Put it on an 18' pole
and cover the pole with anti-climb paint (keep folks honest). Run the wire down to
the
sign lamp. A local light shop can sell you a pretty bright bulb that uses low energy
(12 v DC) and probably the proper reflector to go with it.
The tough part is selecting a battery as they are all over the map when it comes to
prices and features.
Not unheard of is a wind turbine on top of a pole to charge the battery. Lots of new
technology in that area and prices are reasonable.