Ferrofluids are not all the same. Many factors are affecting ferrofluid performance: weight, density... quantity, type and size of magnetic metal particles.
You can make your own pretty cheap. Receipts are simple. Test and class their magnetic and fluidity properties, then compare their heat dissipation property inside a running hub motor, and find the ideal composition for this purpose. Those tests, materials used, and strict composition/production, are for a fair part of the selling price.