I've been looking for these for a long time, and here they are! You can place two canvases face-to-face, or one canvas facing some foam board.
The double pointed barrel-shaped pins are available at Binders
http://www.bindersart.com/store/prod...1/Canvas-Pins/
and the corner pins at Art Express, made by Holbein
http://www.artxpress.com/commerce/ca...=1183638458117