Hi Inge,
About 90% of clay we use are Czech clays, we are lucky to have rich deposits of diferent clays here. There are few small manufacturs who make clays and we buy most what we need for our production. They have quite wide menu to choose from, as those manufactures are small, family bussineses, clays are not so stable as from big German companies and can vary from batch to batch. But we see them as more live...We can quite easily source rough clay,almost for free, but so far we use it just for smaller part of what we do. Still looking for way how to prepare such clay an effective way. We quite offten mix clays togehter, adding some grog or other stuff if needed.
As for the porcelain, we mostly use technical porcelain sourced in faktory which make insulators from it. It is nice to work with and catching flames much better then clean ones. But it is not translucent. Time to time we use translucent porcelain from France (Limoges-TM10), you can buy it in
http://www.wolbring-keramikbedarf.de/ It is closer to you then to us:) It is quite tricky material, but I like it
If you have any more questions feel free to ask!