I don't think you can get it anymore because it was a limited time offer with a money pack that you buy with real money. Only was is to find someone who has it and trade

You can add them by facebook account or pet ID. Just go to Add Neighbours, then Other Facebook Users playing Pet Buddies. Search their facebook name or pet ID

You can get gems by serving hazelnuts and honey roasted peanuts. Also sometimes in the bags that you find at neighbours, the daily ones, and the sleepcare ones.

You can trade with other players on facebook groups