you can get it from some garden nurseries , or just get some from the builders yard , gypsum = plaster. I can't remember which one to get now , but if you tell the builders yard people that you just want a basic gypsum , they should give you the right one. But failing that Linz , my soil was very much clay under about 9" of top soil, I just added plenty of compost and manure . You can also grow a green manure of alfalfa and after it has been growing about 6-8 weeks dig it in . The clay I had was even blue in some big areas ,(that generally means waterlogged airless clay) that has all gone now and the soil is really nice loamy crumbly stuff on the whole. It is also like that down two really good fork depths , so it can be done and doesn't take that long . The gypsum , by the way , makes the tiny smooth clay flat particles stick together, to form a larger more irregular shaped particle. These new particles form what looks like crumbly soil and allows more air into the ground and therefore the roots of the plants.