You've got yourself a really great camera. Here's an article that explains Series filters.

They are sized in Roman numerals. My Ikonta 6x6 with a 70mm lens uses a Series VI. As Peltigera says, that might be the size you need, or maybe a VII. Don't bother looking for anything new as this whole system has been obsolete for about 40 years. You can usually find them pretty cheap at photo swap meets.