The following I found in various threads around here seem like simple, low cost, and (mostly) natural alternatives if you're out of nutrient.
If you are boiling you can add any dry yeast to the boil, including bread yeast. It is the same as using yeast hulls. Yeast are cannibals and they will go for the dead yeast first. -Revvy
use grape nuts it works great, take one cup and add water boil for a couple of min in a microwave, then cool and strain add only the liquid to the fermenter. It will take off like a rocket ship. -yodalegomaster
(I was curious what makes grape nuts a good yeast nutrient so I looked up the ingredients: whole grain wheat flower, wheat flower, malted barley, salt, dried yeast, soy lecithin, other things including 6 kinds of B vitamins. All sounds yummy for the yeasties! I read somewhere they need B vitamins.)
Bee pollen. If you're buying directly from the producer, ask them.
Raisins. Chopped, smashed, or generally abused.
Lemon juice is a good substitute. Just add it to taste, probably 1 mL at a time.
I was curious about yeast nutrient and energizer after I got the stuff I ordered off the internet so I did a little digging. I really didn't like the idea of putting urea and diammonium phosphate (DAP) in something I plan on drinking. My bad for ordering something without reading the ingredients. Thought I'd share what I gathered. Cheers!