Yep - Dawn covered it. That being said, it isn't as available over here
as it is across the pond!
You can order from
www.EcstasyCrafts.com or
www.SweetStamps.com - they carry a lot of pergamano tools as well. I have ordered heavier weight vellum, in a pinch , and it works as well, but you have to watch the weight of it. Pergamano vellum is a much heavier, sturdier weight and I think 48 lb. glama vellum is comparable (going from memory so that may be a bit dangerous!) You need the weight to handle the embossing well and get the good "white" results.
As far as regular vellum is concerned, I love printing on vellum , especially the marble white, and then embossing it. The ink is wet for a short time so you can print a greeting, for the inside of a card, pour clear embossing powder on it and heat emboss. It really makes the printing stand out. If you don't emboss it, you need to be sure and let it air dry a while or it will smear.
Hope that helps.