Guess I’m officially a cheapskate.
I made my own ferrofluid. I have a bag of ferrite cores I bought for making coils. I used a smooth knife sharpening stone and ground the ferrite core into a fine powder. I used 30 wt Mobile 1 synthetic oil. I mixed the powder into 1 ounce of the oil until it was dark like strong coffee. Then I let it sit and settle overnight. The big particles settled to the bottom. Then I poured the clean ferrofluid into a clean glass bottle with a tight cap. I used an inkjet refill kit syringe to inject the fluid into my dome tweets and midranges.
BTW, that was 17 years ago. Mobile 1 is more stable than regular ferrofluid oil; it has heat stabilizers, antioxidants, detergent, in its additive package. Probably time to check my drivers and do a refill. Also, I used contact cleaner to FLUSH OUT the old ferrofluid. Paper tape will not remove all the old gunk.