If the marks don't reappear when in the freezer, you need to lower your freezer temp or get a new one! 14 degrees or lower is the temp the marks will reappear. Your freezer should freeze foods between 5 and 0 degrees. Your fridge should be keep refrigerated foods 34 degrees.
I found them at Sam's Club before they became popular after reading about their quilting use on a quilt blog about a year ago. I got a set of all colors (no white but the orange and pink shows up great on dark colors) for less then $8.