If you have non- tongue&grooved boards, it's likely that it's an older house. The boards would have been Imperial in size. Common sizes will be 6" or 7".
A 7" will be approx 178mm. The modern metric equivalent is around 170mm wide. (After planing)
If you want to replace one, the usual thing is to buy a 200mm board, and either saw or plane it down to width. You would have to pack it off the joists, as modern ones are 22mm thick, and old boards approx 25mm.
A simple solution is to lay the old board tight to one side, with a wide gap on the other side. Assuming the gap to be around 6mm, then cut a strip of 6mm MDF, and glue that into the gap.