All you have to do is 1) identify the city 2) get the ordinance on line or by mail 3) go to the “non-premise advertising” section 4) see if billboards are allowed and, if so, in what spacings, zonings and sizes. If the ordinance says “no billboards allowed” then move on to the next. It should not take you as long as it is taking. You should be able to go through an ordinance in less than 1 hour from start to finish.
You need to build a “territory” of roughly 4 hours drive from your house (draw a circle around your house 4 hours out). This should contain a number of legal cities and towns. It sounds like you simply need to expand your territory.