Ideal standard toilet - side action cistern lever ?

The fitment nut and bolt on my side action cistern lever has corroded and broken.
The unit is 19 years old.

The plastic fitment fitting onto the lever is OK.
The lever is OK I just need a new nut and bolt to get it working again.
I have tried hardware stores but no good.
Where can i get such a nut and bolt?

Its 17mm long, 6mm diameter fitting into a square nut 12 mm square with a decorative chrome header on the bolt head