I am pretty sure that it is completely random. Try resetting the time to refresh the shop stock! It might increase your chance of finding one. Hope that helps!
Sorry, but you can't "make" it come. You can't get it in TamaTown, either. Just wait for the shop to restock, but I don't really recommend changing the time...if you change the time over and over again it could mess up your Tam, I think. Just wait for it to come. It came in my shop and I got it. Until then maybe you can start saving up Gotchi points. The UFO is really expensive. I got it for 50000 Gotchi points and I think it was on sale!