Hello Carld, i´m from Germany and interested in mulberries too. In searching for Gerardi Dwarf did you find this shop from Poland?
I ordererd a Gerardi Dwarf and a Shin-Tso cultivar. If everything works out they ship in autumn. If you have the patience, we can swap scions later on. But it might be a year later since i dont know the size of the trees. They are shipped in 2l containers.
With the kind help of another grower here in Germany i got my hands on a morus nigra "Jerusalem". And i grafted 2 different cultivars of morus nigra, which i brought back from Gran Canaria in may. The motherplants are 2 very old and quite big trees. I suspect those are seedling trees, but both had a lot of immature fruit. Hoping to get a first taste next year.
To save your old tree try grafting low onto cuttings of morus alba and root those (or root the alba first, both ways are possible). To get this tree true to the roots you can then later on plant the tree deep. With some luck the scion will develop own roots. That happens quite often by accident, when people are planting their trees too deep. In those cases its an unwanted process but you can use it to your advantage.