I have purchased a few of them from Columbia City Collectibles. They always seem to have a pretty good inventory of odd ball advertising items.
http://www.columbiacitycollectibles.com/
I enjoy the larger Spalding advertising pieces. Unfortunately, they don't seem to show up very often. A couple of years ago I purchased this early 60's Drysdale POS advertising poster from Columbia City Collectibles. It measures 11 X14. Despite an extensive search I have never come across another example of one. In 2011 Heritage auctioned off a similar larger 3D version that they attribute to 1962. However, they were constructed of thicker stock cardboard.
http://sports.ha.com/c/item.zx?saleN...&lotIdNo=21001