You're probably going to have to buy a blank and make your own. A slotted nut, if you already have one or are willing to buy one, will make a good template to speed up the process of shaping the blank. Here's one that I found, but I suspect that it is not high enough:http://www.graphtech.com/product-299-2/
You can buy larger sized blanks, but these will be white in color and more than 1/8" thick.... you'd have to sand down one of the sides to make it thinner. The one I posted a link to is available from several sources, and for less money. This ad had the most descriptive information.
You could get some similar nut material (an old, no longer needed nut) and scrape/grind it to get shavings/small particles; mix these with Super Glue, and fill in the slots on the nut you have. That's all I have....
BTW, what are you up to?