Forgot to add... When I bought my 2 data plates for my '55 C3B, back in September, I was told they were on back order. I placed the order and they showed up in about 4 days.
Try Vintage Power Wagons (http://www.vintagepowerwagons.com). They don't show everything on their Web page, but I see it on p.88 of their catalog. It's item #53 for $8.50. Bob