I think Woodford garage are the only ones of the top of my head, who still have them in stock.
There is likely to be someone on here who will have a used but in good condition one.

Not sure myself what way the water goes round.

The flush stuff would be a good idea but I would just for now, remove the radiator, flush that out until your happy that is clear and then if that doesnt fix it then use the flush stuff. It should break the sludge etc etc enough to not cause damage to the pump.