Adding a description which appears when product is sold out.

[maurice4]

Hi there, 

We have our online-shop with shopify quite a while now and are facing a little issue at the moment. We are selling books and magazines and have a physical store as well. Our stock is very limited and we change the selection constantly. To give the customers the chance to order books that we don't have in stock anymore we created a second website from which they can order directly from the distributor. 

Now the issue: We would like to display a message saying "Order this book from our second online-shop" which appears once a book is sold-out. I couldn't find any app so far that is able to do this. 

Maybe someone can help?

Thanks in advance