I have a toilet seat and cover that are connected by a rod running
through them. It is about six inches, say 15cm long.

The original flexible plastic rod broke after 30 years. Th nearest
replacement that I could find was acrylic rod.

I have had two so far but they crack after about a year.

I would consider a stainless steel rod if I could get one of those
dimensions (150mm by 9mm dia) but would prefer some kind of flexible
plastic.

Can anyone recommend a source?

TIA