Dwarf and semi-dwarf cherry rootstock is very hard to get in small quantities. Take a look at this thread:
Your options basically are:
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Buy finished trees on the rootstock you want if you can find the cultivar/rootstock combination you want.
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Produce the rootstock yourself using stooling techniques for the rootstocks that are off patent. You will still need to buy at least one tree to start stooling. Or you can also induce suckering of the rootstock of the tree and either reproduce the rootstock by taking cuttings or removing part of the sucker with root hairs. See the link within the link above for info on taking cuttings. Also this is a fairly long, slow process.
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You could approach vendors that graft and offer trees on Krymsk 5 or 6 rootstocks. Ask them to sell you a few rootstocks directly. I don’t think this approach will work with the Gisela series due to how these rootstocks are licensed.
You probably can buy the Gisela series if you are willing to buy 50-100 rootstocks and jump thru some hoops but I don’t think that is practical for most people.