public sealed class ClassifierProvider : MarkupExtension
    {
        public string Classifier { get; set; }
        private Dictionary<string, object> cash = new Dictionary<string, object>();

        public override object ProvideValue( IServiceProvider serviceProvider )
        {
            if (IsInDesingMode() || String.IsNullOrWhiteSpace(Classifier))
                return null;

            object value = null;
            if (this.cash.TryGetValue(Classifier, out value))
            {
                return value;
            }
            else
            {
                var list = new ObservableEntity<ClassifierInfoList>( );
               //Здесь должна быть твоя логика для получения данных
                this.SetCash( Classifier, list );
                var load = list.FetchAndSubscribe();
                value = list;
            }

            return value;
        }

        private void SetCash(string key, object value)
        {
            this.cash.Add(key, value);
        }

        private static bool IsInDesingMode()
        {
            return System.ComponentModel.DesignerProperties.IsInDesignTool;
        }
    }
}

	<local:ClassifierProvider x:Key="UOM"
                Classifier="Единицы измерения" />

             	  ItemsSource="{Binding Source={StaticResource UOM}

  public sealed class ViewModelProvider : MarkupExtension
    {
        public //Type 
            string Type { get; set; }

        public override object ProvideValue( IServiceProvider serviceProvider )
        {
            if (IsInDesingMode())
                return null;

            if (this.Type != null)
            {
                var xamlTypeResolver = serviceProvider.GetService( typeof( IXamlTypeResolver ) ) as IXamlTypeResolver;
                Type type = xamlTypeResolver.Resolve( this.Type );
              
                var entityType = typeof( ObservableEntity<> ).MakeGenericType( type );
                var entity =(dynamic)Activator.CreateInstance( entityType );
                if(entity == null) return null;

                entity.FetchAndSubscribe();
                return (object)entity;
            }
            return null;
        }