where is this helmet located?

Sorry for the late reply, I don't think you can buy helmets (the character appears to just put a random one on when you jump on a motorbike) I used a trainer to get mine (https://www.gta5-mods.com/scripts/simple-trainer-for-gtav All credits to sjaak327)
if using the trainer it will come under 'hats' and it will be as the following:
Franklin - Helmet 18
Michael - Helmet 24
Trevor - Helmet 24

@BULLFAYCE There is a mod menu which give you the possibility to add Helmets,Masks etc.... Here is the link: https://it.gta5-mods.com/scripts/helmet-switcher

thank bro. the mod menu works fine. light a blunt to that!!

@OnSuorce you are stupid it is the same page n which it wrote idiot