I'm sure he has someone else make it for him. Or maybe Snape makes it for him all the time. We know very little about what the teachers do outside of school or during holidays. Prehaps Snape makes barrels of the potion so Lupin always has a supply at hand.

I don't think Lupin is using the potion very much though. He is currently assigned to undercover work with Fenrir Greyback and his gang of werewolves. This would also explain his looking ragged in HBP.