I dyed some DK weight, Superwash Merino yarn in a semi-solid neon pink called Scrunchie which is a very bright "need your sunglasses" fluorescent pink. This colour will fluoresce under black light.

Classic DK:

100% Superwash Merino
~250 yards
DK Weight, 3-ply
115 grams

For best results I recommend handwash cool, dry flat.

The cost is for one skein though multiples may be available - please select your quantity from the drop-down menu.