I bought a Summarit 50/1.5 last December and had the same problem, where to find filter ?
First attempt: I bought two Kenko filter (Japan), 41mm (one UVa and one Skylight 1a). The UV filter fits, the Skylight does not (different pitch)
Second attempt: Buying a step-up ring (custom-made, 41 -> 43), the pitch of the thread does not fit.
Third attempt: Buying a Tiffen series vi adapter with 41mm thread. The thread diameter was to small, despite being marked as 41...
Fourth attempt: Using a standard 40.5 UVa filter from my 40/2 M-Rokkor CLE. Fits perfect, no vignetting and I can use a 40.5 screw-in hood from heavystar (wide version) with hood cap. So far the cheapest and best solution.