I assume you mean the bits outlined in blue? Including the pocket itself (which is integrated into the leather loops).
These are often called stashes, holster bags, pocket belt or pocket bags, or utility pockets or utility belts. See, for example, Verillas's "Gunslinger stash"
https://www.verillas.com/gunsl.....d-edition/
and this "pocket belt bag".
https://www.etsy.com/listing/7.....ap-batwing
You're most likely to find them from places focused on punk, goth, RenFaire, or cosplay-type gear.
