I know you have a Wall Mart somewhere in Oregon. Should be available in small quantities there. Buy a bit, give it a good whiff and then give nutmeg a good whiff. You may or may not be able to smell a dramatic difference. I find Mace to be more savory/spicy than nutmeg.
If a recipe said you can swap coriander for mace you are really making a big change in the flavor profile of the sausage.
Apple Pie spice should be reasonably spiced and is a mixture of things like Cinnamon, mace, nutmeg, allspice, clove etc. I would go with that before coriander. Or any of the components of apple pie spice.
Mace and nutmeg can get bitter when used in quantities greater then 1 gram per kilogram of sausage so unless you are making a great deal of sausage small lots should be sufficient to allow you to develop the recipe. Marianski says that one flat teaspoon of Mace weighs about 1.69 grams