We cannot recommend contractors on this forum but we can provide a link to a web site that explains how to hire a certified arborist.
Here is an Extension publication on how to hire an arborist: http://www.extension.umn.edu/garden/yard-garden/trees-shrubs/how-to-hire-a-professional-arborist/
You may also want to check with your city as municipalities sometimes have arborists on-staff.
Part of controlling apple scab is good sanitation. Removing all the fallen leaves from the crab apple tree this fall and mulch on top of all that can't be removed will
greatly reduce the infection next year. Also prune your tree to increase air circulation.