I was in your situation and used these. Cheaper and I believe better quality (A4 stainless) than the originals!

I found this at the time and went by the scale in one of the pics, I can't find a receipt, but by the pic I think I would have ordered the 30mm ones.

As for genuine parts, I use fowlersparts.co.uk for the parts fiche though I've not actually bought any parts from them.

Welcome to the forum!