Look for FSU sellers on eBay, with solid feedback.
When I built my FSU camera collection, I was able to buy many of these for little cash and all were in good shape ( none of mine are for sale) and delivery was reasonable.
A side note; I once bought six Fed LTM rangefinders and they showed up in a canvas bag, like lumpy potatoes, with gritty wax seals and yak hair string stitching the bag shut.
All were working and I did several more deals with that seller, but never bought enough again to rate a second canvas sack.
If you're still looking, and have time to wait for shipping, these are available from various eBay sellers for as little as $4 each plus reasonable shipping. I've ordered sets (by the single filter) for my 40.5 Kiev lenses and my 77mm RB67 lenses, and so far I'm very pleased with them. They're not marked with a Wratten or other standar filter number, but clearly marked on the ring for color.
One last option for this old thread: If you have Series VI (6) filters, buy a Tiffen 602 adapter. It screws into the 40.5mm lens thread and holds the Ser. VI filters.