If you can't get a replacement from the Manufacturer, the best and cheapest option is to locate a shop that rebuilds pumps for the agriculture business. If you live in an area with any agriculture, look for "pump repair."
Little Giants are widely used in landscaping applications too. Years ago our families business did a lot of these for ponds and waterfalls. I would think you just need to match the flow and pressure to what you need. You might check landscape or garden, nursery supplies if you can't get it rebuilt.