As Jack said, ^^^^^ Crabs are really good pollinators for regular apple trees. Adams County Nursery ( ACN ), sells crabs just for that purpose, along with their regular apples. ACN puts a chart in their catalog that's a pull-out. It lists when apples and crabs bloom and which ones will be good pollinators for each other. Even if you don't order any trees from ACN - it's worth e-mailing them to have a catalog sent to you. Save the pull-out for future use. ACN also has peaches, plums, cherries, apricots and pears. FWIW.
Adams County ( Pa. ) Nursery is listed on many university sites and is one of the top rated fruit tree nurseries in the whole country. They are a research nursery as well, with newer apple tree varieties tested there before being released to the public.