Fons and Porter call their's Quarter Inch Seam Markers. There are two sizes in the package and I think that you can find them at Joann's. Here's a link
http://www.joann.com/search/_quarter_in_seam_marker/
You could also draw a line diagonally from corner to corner and then sew 1/4" on either side of the line.
Good luck!
Colleen