Do you know what infected your previous ash trees? That may or may not make a difference with the next trees you replace them with. If your previous trees were cut to soil level but the roots are still in the ground, their biodegrading will inhibit the roots of the replacement trees for a period of time. New trees could be planted at any time, but are likely to do best if planted in the spring or fall. 
This web page is good when you plant new trees.
https://extension.umn.edu/how/planting-and-transplanting-trees-and-shrubs
We cannot recommend specific companies by name, but if you do a search for tree nurseries or garden centers in your area that sell trees, you should get a list of sources.