The only Loop that Apple sold separately were the Jam Packs. You stil can get them on Amazon but you are getting them much cheaper as part of the MainStage application. It comes with 20GB of all the Jam Packs.
You are aware though that FCPx cannot utilize Apple Loops like GarageBand or Logic. FCPx (norFCP7) uses a Tempo and Key parameter in its project and therefore only utilizes the Metadata of the Apple Loops but not the Follow Tempo and Key feature.
If you had FCP7 installed and all its content (Soundtrack Pro Loops) then FCPx will recognize those APple Loops and let you search for them (including Metadata) in the Media Browser.
Here is a screenshot from FCPx's Media Browser displaying those Apple Loops. All The GarageBand and Logic Loops are not displayed, however you can drag them from the Finder onto the TImeline.Hope that helps
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