Yeast nutrient is there to help the yeast survive and multiply and stand up the the high alcohol.
I got my yeast nutrient at the home brew store. it was 1 tsp per gal when you pitch the yeast.
If you dont have a home brew store near ya...Go online and search for best price...
so, this is my understanding of the nutrient..If i am wrong someone wiser than me will chime in