The MS Store for Business is free - you may already have access via an O365 account if you or your organization has a subscription (but in that case you may need to request an admin in your org to add the App Installer app to it if you are not a global admin yourself). See this article for more info on downloading offline app packages from the MS Store for Business.
The MS Store for Business also lets you download any dependencies you need for a given app. Just download the offline bundle for the App Installer app, and install it, then you can use it like normal to install.
If you don't want to use the unofficial workaround to add the MS Store back to the LTSC build (since some have reported that not everything in the Store works properly when it is added to an LTSC build using that method), you can just use the MS Store for Business instead.