That part wouldn't have a proper name industry wide. It is part of the LNB/Feed support assembly on manufacturer specific dishes. Mostly used in a professional application and not the common residential C-Band dish. I have seen these on big dishes at Cable providers and TV stations around town with different designs but providing the same function.
i can think of 2 possibilities:
- browse the sites of professional-level dish manufacturers, looking for parts or accessories. Might be quite expensive just because they can.
- fabricate one. Might not be easy to make by hand because of the elongated holes, but not impossible. As RimaNTSS mentionned, a laser cutter would make this quite easy.