Hi Mary -- Too bad about the nearby crab apple trees! That said, place a pollen source within 100 feet of your tree to ensure the pollen reaches it. Apples are insect pollinated, with bees and flies transferring pollen from flowers of one tree to another.

To cross-pollinate your tree, plant other apple varieties that bloom around the same time as the Harrelson. Some apple varieties that could serve as cross-pollinators include Honeycrisp, Haralred, Sweet Sixteen, State Fair, Keepsake, Frostbite, Cortland, Zestar!, and Paula Red. You could also plant your own crabapple. Chestnut crabapple is a good one. The fruit has an intense nutty flavor and it is moderately resistant to apple scab and fireblight.

Information on some of these varieties can be found at:  https://mnhardy.umn.edu/varieties/fruit/apples/all-apple-varieties
and 
https://extension.umn.edu/fruit/growing-apples#apple-varieties-recommended-for-minnesota-247061 

Good luck!