I dyed some fingering weight, Superwash Merino and nylon sock 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.
Basic Sock 50-gram mini skeins:
80% Superwash Merino, 20% nylon
Fingering Weight, 3-ply
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.