Apple has updated the Apple Store app for iPhone and iPad, bringing support for Siri Shortcuts. Once you have updated to the new version of the app, you will be able to configure Siri to check on product availability and on your order status without having to actually open up the Apple Store app first.
As with other Siri Shortcuts, setting them up in the Apple Store app is pretty straightforward. For the product availability feature, open the Store app then go to the product you are wanting to find. Next, select the Store location where you want to check. At the bottom of the page, you will see the option to create a Siri Shortcut to check for that product at that store.