Hi Urban,
For the 58mm filters I use with the mamiya I sourced them all used and new!
hoya, vivitar, jessops, Victar, quantaray.
i have a good selection, R72, Red, Orange, Deep Yelow, Yellow, 81A, CPL, UV.
it takes time if you want to save money and buy used.
Marumi filters [new] are a V.good brand with high coatings if thats what you are after
and would be cheaper than HOYA.
You could try ebay, ETSY.com and APUG !