I bought mine from Aircraft Spruce...
...called Jack Extensions.
I used the 3" extensions, p/n 12-00157. In retrospect, I could have used the 1.5" extensions.
The extensions slide over the jack point on the 3-ton bottle jacks I have, and because the jack extensions are abt 3' deep, I need to unscrew the jacks' threaded jack point about three inches so that the lifting surface fits up inside and bottoms out inside the extension.
If you go this rout, also be sure to obtain a set of "collar clamps". They need to fit around the jack's hydraulic ram after you get the jack jacked up. You then fit the collar clamp around the exposed ram and clamp it down with the provided screws. This prevents the jack ram from retracting if it should loose pressure. Your jack ram diameter needs to be measured, and order the size that fits. I got mine from McMaster Carr