Needed: Old steel guitar strings

My daughter has picked up an old instrument from a charity shop. I don't know what is it; it can be a Zither or a Psaltery! It is a 12-string instrument (currently 10) but there are two missing from the high end. All the existing strings are the same thickness!

I have been looking to find a complete set of similar strings but haven't been able to find any. I was wondering if anyone has a length or two of old steel guitar strings (top E probably) so I can put it on the instrument and bring it back to life!

Thanks in advance.

Ali