You can graft it onto hawthorn, that might be a cheaper and faster method than planting.
Also many hawthorns are hardy to Zone 2 and may make your Medlar more cold hardy than whatever rootstock you buy it on.
Also try getting Arnold hawthorn! You might like it better than Medlar, it’s more like big blueberries than anything else, but has big seeds too, very productive in zone 4, hardy to Zone 2.