You have two choices, really. Do you want a truly awesome gooseburger? Then buy a domestic goose, cut out lots of the extra fat (there will be tons) and use that mixed with the goose meat in an 80% goose, 20% goose fat mixture. If you want a nice gooseburger but not quite as good, buy pork fatback and do the same thing. One word of advice: KEEP THE MEAT AND THE FAT COLD!!! If it gets much beyond 40 degrees you get "smear," which is the fat melting. Your burger will then be dry and unhappy. Keep the meat/fat in a metal bowl over another bowl filled with ice.
This should start you off right. Let me know how it goes!
I love all of God's creatures, properly prepared.