Osmocote pellets are one accepted fertilizer for Nepenthes. I do not have experience using them with anything else.
Some place a few pellets in the media of the nepenthes, others put them in the pitchers. I have only put them into half-dead pitchers, so use at your own discretion. Too much fertilizer can cause pitchers to burn, so it’s best to put them in half-dead ones as they are already dying and still absorb from the bottom.