40mm filters are scarce as hen's teeth, unavailable at the usual local or in fact any N. American sources. Kudos to Fotoimpex & Heliopan reasonably priced & just a few weeks order time....delivery via DHL. Medium yellow 40mm for my Canon 28mm f2.8!
I have a drawer full of Heliopans. The real tip of the hat goes to Fotoimpex who can supply a 40mm filter (not 40.5 !) when B&H, Freestyle, Midwest Photo & The Camera store don't even list 40mm! The Heliopan quality is a + for sure!
I'll second that. When I first started serious photography, I was living in Germany. It was so easy to get Heliopans there and they were quite reasonably priced, and the local camera shop recommended them to me. I've still got those 20 some years later.
Besides their filters, their step-up/step-down rings and their reversing rings for macro photography are excellent, too--made of brass rather than aluminium, work brilliantly. They're the ones I use for those purposes.