A content instance that implements EPiServer.Core.IResourcable stores related assets in an instance of EPiServer.Core.ContentAssetFolder. Resources that are stored as content assets are considered exclusive assets for that content instance, and therefore you cannot select them from other content instances.
The following example shows how to get an existing assets folder, or create one automatically:
contentAssetHelper = ServiceLocator.Current.GetInstance<ContentAssetHelper>();
var contentAssetFolder = contentAssetHelper.GetOrCreateAssetFolder(myContentLink);
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