Ladders you can get from Plastruct...if lucky, from your LHS, which should also, but probably won't, they are always out and never restock, have the stripwood...I bought a billboard kit off eBay, found it simple and now just adjust the plans to make whatever size I want to fit the poster...but you can design your own from the examples posted above. You can also get the stripwood from MicroMark, or, if lucky, from Hobby Lobby or maybe Michael's...artboard/carboard back, stripwood posts,
braces, and trim...paint, use rubber cement to glue on poster...Voila'!...billboard..