Your best choice is to buy Krymsk 5 from Grandpa’s Orchard.
https://www.grandpasorchard.com/Tree/Prunus-rootstock-Krymsk-5-Semi-Dwarf-rootstock
The shipping calculator doesn’t work correctly for rootstock only orders as per their note on there website.
“SHIPPING CHARGES NOTE: We will usually adjust your shipping charges when ordering rootstock by itself as our shipping calculator does not calculate correctly for rootstock. When ordered with trees, we can ship in the same box for no extra charges. When ordered by itself, it may ship for less. If you have questions, contact us.”
For more information on the dwarf and semi-dwarf rootstock situation look at this thread.