For stock RD parts, RDLC Crazy is a good place to take a look. For awhile it seemed like Norbo (the site owner) had quit selling RD400 parts. Apparently he redesigned the site and the same parts are still for sale. It’s just harder to find the links now.
Here’s the direct link for RD400 parts.
And the direct link for the RZ350 (also known as the YPVS)
He’s in the UK, shipping will be expensive compared to buying parts in the U.S. On the other hand, this site has parts that I haven’t found anywhere else in recent years.