I think you are too late for rootstocks or trees at this point.
Really now with the pandemic you want to start looking for rootstocks and trees in November and have a order in by the end of December. Dwarf or semi-dwarf cherry rootstocks are hard to get in small quantities. I would suggest buying trees instead at least initially.
Raintree nursery offers trees on Gisela 5
Trees of Antiquity offers trees on Krymsk
Cummins Nursery and Grandpa’s Orchard offers trees on Gisela rootstocks sometimes
Another option is Schlabach’s. They have a fairly good selection of sweet cherries on Krymsk. My trees from them came on Krymsk 6. Trees run about $20 ea.
They are Amish so you will need to write a short letter asking for a catalog and send them a check for $2.00.
Schlabach’s Nursery
2784 Murdock RD
Medina, NY 14103
For rootstocks look at the link below. It has some ideas on what you can actually buy. One thing to remember is Krymsk 5 and 6 cannot be used for multi-graft trees since they are sensitive to latent viruses. In addition to the sources in the link you could ask if you could buy some Krymsk from Schlabach’s Nursery. For Gisela 5 you’ve going to have to propagate it from cuttings (it’s off patent). Details on how to do this are also included in the link.