Matthew:
Over the years I have owned 2 billboards. Neither of them were digital but both on main road ways with great daily car counts.
In my experience most of the ones I have marketed to are not on an easement (separated from the main property it is on.) So what I did is drive around and got the address of where the billboard was located and mailed the owner of that property.
As far as returns, that is really up to you as far as what type of return you want each year based on the cash flow that the billboard throws off.
In my experience the current owners don't push for the max they could get from a cell tower of billboard because it usually not their primary business, just a extra income source.
I would research your State Laws on billboards and see what you can and can't do first. I know here in Michigan they are not allowing any new permits for billboards, last I checked. And then if you do find an owners who wants to sell you their sign, you have to make sure that you can get an easement (separate parcel ID for the sign itself) from the other property it is one. Now, if the property has a mortgage on it, getting the mortgage company to break that part of the land off the mortgage can be a trick.
The cash needed to invest varies, based on the seller and what returns you want to obtain. I paid several hundred thousand dollars for both of mine. I don't remember the exact numbers, sorry.
Also look for billboards owned by companies like Lamar because they typically will pay a higher monthly payment.
To find them: Check out this site for a place to start building your list of signs. Check those for sale as well as lease. http://www.outdoorbillboard.com/ and drive around your area, mostly by freeways and obtain addresses. Then go to that city/town's online property tax lookup and find the owner from there. Now, most of the properties may be in a name of a LLC or Corp. so from there go to your state's corp lookup to get the owner of the corporation.
Hope that was not overload. Let me know if I need to cover anything more.
Hope that helps,
Pat Gage