where can I purchase the hardiest variety of crepe myrtle?
I would recommend visiting Petitti's. That will be the best best in this area for finding a hardy crepe myrtle. Even then, there is no crepe myrtle that you can find that will grow well in the NE Ohio area. It is possible to grow one, but it will need a sheltered location and even then, it will die back to the ground every winter and will never be more than a shrub.
I would recommend looking at an American Fringe Tree as an alternative. It is hardy in this area and will give you a lovely show in the spring. You can also look at Pagoda dogwood (and many other types), Japanese lilac, flowering crabapple, viburnum, serviceberry, and redbud.