Hi I have a Volvo 340 1989 3door , has some rot in the rear inner sill

is it possible to buy inner sills?

Thanks

I don't think I've seen inner rear sill panels before, usually these will need to be fabricated - but they tend to only need fairly localised repair and don't need to be that pretty!

Inner sills where only made as a complete section (going around the complete door opening).
I have not seen one in the last 25 years, not exaggerating (!!), so your best bet is to make a repair pannel yourself or get a part from a scrapper.