You can use a fatty cut of pork (such as pork shoulder) or you can look for unsalted back fat, or even uncured belly to mix with your deer. If you are grinding everything together, just cube it all and send it through the grinder together. If you are making sausages, you should also mix it well for several minutes until the meat has a "bind" or starts to stick to the edges of the bowl.

I have used commercial meat grinder, and it works brilliant.