Concentrated clay lube refill. Dilute 1:10 with water.
Born Slippy is not only highly lubricious, it also enhances the cleaning process while you clay. And when sold as a refill and correctly diluted, it's cheaper than a lot of trade products out there many of which aren't even made for the job.
At 10:1 as a dilution ratio, one 250ml bottle of refill could make over 2.5 litres of clay lube. It could be even cheaper than using a strong shampoo solution.