Lee Valley sell Hirsch Mortice chisels which I assume are GErman and therefore available over there. I have not as yet tried one but I have to say they are the biggest, ugliest chisel I have ever seen probably work well.
I have a Sorby 3/4 inch that is frankly not that great doesn't hold an edge. I also have a 5/16, I think older Marples the kind with the red and yellow handle and it is very good and as Alf says you can get these fairly cheap on Ebay, I recently used it for doing two inch deep mortises in white oak and have no complaints.