All of the auto parts stores carry the shim assortments (if you need one). Yes, they DO adjust the started vertically but that's just as good as adjusting them horizontally because of the starter's 45 degree location in reference to the ring gear.
There isn't any "in and out" adjustment. As long as the Bendix retracts completely out of mesh with the ring gear teeth that's all that's needed. And as far as going in too deep the face of the Bendix stops it.
To check a started for it's adjustment pry the Bendix into mesh with the ring gear teeth. Once fully meshed the gear should be able to be wiggled a little bit. Some folks use a small paperclip inserted between the teeth but any amount clearance is enough. You'll know when they need shimming as they'll make a LOT of noise.