If a user searches for a product your store doesn’t have, showing an empty page is the worst thing you could possibly do. It leads them nowhere and causes frustration. Frustration can make them bounce from your store.

Include some of the following components

Alternate products similar to one searched for
Category suggestions based on what is searched for
Personalised product recommendations baked on user profile and previous purchases
Popular products and categories
Contact information (phone number, email, chat) and other help links

A different perspective

Below are some thoughts from one of the members of my Ecom Convert group:

I don’t care for this. I’d rather see something that says in a friendly tone ‘I’m sorry we didn’t find that item. Would you like to talk to an agent to see if we can help you find what you want?’ Then a live chat link or offline contact us – bot…

It could be live chat or a phone…email if offline but really instantaneous would work much better

That would add an extra level of service as well as provide more intelligence on what people wanted and didn’t find. While you can get some of that from higher end search engines, what this would do is give you a second shot.

Perhaps they searched for it incorrectly and you have what they actually want. (this then helps you optimize your search engine keywords). Perhaps you have a great substitution. Perhaps this is something you should add to your catalog.