Also looking for male snivy with substitute so I can pass down the move. I wasn't able to get the TM last winter.
You can just change the date on your DS (the seasons change at the start of every month, going in normal order) to make your game be Winter again. You can change the date back afterwards if you like. Substitute (TM 90) can be found on Twist Mountain.
Note: It may take a little time for the date change to sync to your game.