If You need to buy Knudson C now- You can try at Duchefa and other TC chemicals distributors, mayby sigma aldrich also have it in offer. It is used for orchids seeds germination (oryginal 1946 formula), it doesn't have ANY iron and vitamins inside, so should be supplemented with MS vitamins, iron, and microelements before use. There is also Morel modification of KC medium- richer one version, containing also NH4+ ions- more widespread in offers, but not especially good to nepenthes ;)
But- Morrel modification ALSO should be supplemented with vitamins and microelements, depending on source- it can have iron already inside, or not.
We use 1/3 MS as a "gold standard"- everything grows well on it. 1/4MS starts to be too poor in phosphorus, 1/2 MS have too high NH4 concentration, and is reccomeneded only to "true" lowlanders.
If You want to try with Knudson C- not important what version- we can prepare it at greenspot.pl for order, but there will be 2 stock solutions to mix [due to suphlates and phosphorates precipitation in single one 10,0 stock]