Here you will find all refills for your Montblanc Fineliner. Make sure you order the right refills! Do you still have an old refill? Then read what it says on the refill.
Does it say 'Fineliner LeGrand Refill' on the old refill, then you have a Montblanc fineliner in a larger version and so you specifically need the LeGrand refills for this. Does it say 'Fineliner Refill' on the old refill, then these are the 'normal' fineliner refills from Montblanc.