How to display 'Only Available by Custom Quote' instead of 'Out of Stock' for certain products?

We have products that are available for online ordering and others that are only available as custom orders. All of the products in our Shopify store are enabled to Request a Quote.

Our ‘custom order’ products are listed in the store with pricing but they are all set with inventory at zero. So they all display as out-of-stock. We want the ‘custom order’ products to display as ‘Only Available by Custom Quote’ in stead of ‘Out of Stock’. The products that are available for online ordering should still display ‘Out of Stock’ when their inventory gets down to zero.

What the best way to do this? I was thinking that I would need to create a new product page template but not sure if that’s the best way to achieve the goal.