Johnson was influential, but he was only one of many artists who did banners for sideshows in the first half of the 20th century when they were at their height of popularity, and the work of some of those other artists rivals or even exceeds Johnson's in quality, though none were as prolific. And with the revival of the popularity of sideshows in recent years other artists have joined the tradition, either doing new posters for new sideshows or adapting the style to other purposes.
Paul Sautzer has done some very clever modern sideshow banners, including a series for the amateur carnival which accompanies the Mount Desert Island Marathon every year. A company called Bandimals will design and produce custom banners in the traditional style for any purpose at relatively reasonable prices. A lot of good examples of contemporary sideshow art can be found at SideshowWorld.
If you like folk art with a long history and interesting subjects, you can always wait for a local gallery to put on an exhibit, or the next time a carnival comes to town, go down and take a look at the banners at the sideshow. They may be more of a revelation than the often shabby wonders inside the tent.