I always use Cimicat, it is brilliant to use for lactating females and also if the litter is too big or if Mum isnt feeding properly etc.
Its is powdered like baby milk and has a longshelf life,so if you dont need it it will sit in you cupboard till the next time.
Cimicat - Milk For Cats
it has a couple of different packaging depending where you but it
Cimicat (350g) - Paws247