I only need to make the texture of the rope of a lead fully transparent, but I can't find the texture. :/
Can anybody help me?
There is no texture. It is and always has been hard-coded. Same for the fishing line.
Optifine used to be able to change these things, but last I check the more recent versions have not included this functionality. Sadly that means that there's no way to change the lead line texture presently.