This doesn't give you a supplier, but it might explain how to attack the problem.
I had looked into this many years ago when I had a ferrofluid tweeter dry out and I could not find a source in less than 55 gallon drum size.
The ferrofluid recipe is not complicated. The ingredients are:
(a) ferric oxide
(b) oil carrier, typically petroleum
(c) surfactants to reduce the surface tension and improve adherance to the structures being cooled, probably a detergent
(d) antioxidants
The oil must have reasonably low volatility, else the heat in the voice coil will evaporate it, and must have thermal stability, so it won't polymerize. I suspect it is a generic refined petroleum oil like mineral oil. Vegetable oils with a high flash point, such as sunflower or safflower at about 500 degrees F, could be used, but even unsaturated oils will crosslink with heat and oxidation, and eventually go rancid. Bleh. Petroleum will also oxidize, hence the antioxidants which are commonly added to engine motor oil.
NB: water can be used as the carrier, but this promotes corrosion and would not be suitable for this purpose.
The viscosity is therefore likely adjusted simply by the amount of oil present.
The question, of course, is what kind of oil is in the original. You might be able to find out what oil was used. I suspect it is a lightweight mineral oil of some sort. Experiments might bear this out. If you know anyone with access to a mass spec you could determine this.
So, you might be able to adjust the viscosity to suit your application by adding additional mineral oil.