There are some on ebay from time to time, but they usually start at $400
I had the same challenge a few years back and I made my own.
Bought a few temperature stable toroidal cores and wounded them myself with good quality insulated copper wire.
Then I had then certified at a calibration lab.
They turned out to be extremely stable in regards to inductance, even with changing temperatures.
Using a toroidal core is the only way, that I found to get them stable and this way, you can even mount
the inductor in an aluminum housing, without influencing the inductance at all.