If you are near a Trader Joes's, they carry one that has gotten a good rating on Serious Eats.

My gingerbread recipe is similar to the one Soozll linked, but it has 1/2 cup molasses and 1/2 cup strong coffee or espresso. The addition of coffee adds great flavor, makes it dark and strong, and is really worth considering. I'm sorry that I'm not in my kitchen at the moment to give you the full recipe.

Whoops...forgot the link! Here you go!
http://allrecipes.com/Recipe/Moist-Tender-Spice-Cake-Gingerbread-Variation/Detail.aspx

Here's a link to a scratch gingerbread spice cake, Check out the which spices are used along with the molassas. I'd be tempted to use the same spice mixture that is used in your favorite, or any other, gingerbread recipe and maybe double any that are used in for single 9inch pan recipe.

Emily, you could certainly do that, but what about the recipe makes you want to use a mix? There are a lot of great easy cake recipes on this site that you could easily add spices to. You could start with the cake from this recipe-- http://www.food52.com/recipes/7569_ginger_apple_torte
just leave the apples out and add the other ingredients. (It looks like a long list of ingredients, but it's mostly just spices. ) If you do that, please let me know how it turns out!