Help matching old creasing tiles?

Hello all,

our old window sills are made from inset creasing tiles, concrete with a greenish colour coat of some sort (the same colour as the original concrete roof tiles although those have lost colour over the 40 years).

Here is a photo:

For the extension we want to match these tiles so that the windows on that wall can all end up having the same appearance. I really have no idea where to look for such tiles - the two local salvage yards haven't got anything the same.

Any ideas? Any knowledge of products that are a good match to these?

Michael