Steve… I’m also a BeCU paddle builder, though not as prolific as @wG0AT .
My source is Digikey.com … Fingerstock I use most is Part number: 903-1405-ND though I also bought some: 903-1398-ND once.
I believe it’s been mentioned to be available on ebay from time to time as well.
Here’s a pic of my KX2 front mount paddle. 3d printed and with BeCU.
Tom - KB9ENS