You might be able to buy something from your local electrical utility company - or from their supplier. They use these fluids (by the barrel full) to immersion-cool high power transformers and such stuff; Midel is one (of many) commercial products engineered for that application. Flourinert is more of an hauteur de machine cooling fluid designed for elite supercomputing applications, stupidly overinflated price like anything else 3M branded.
Midel is about $8/gallon, I think. How much does purified veggy oil cost?