You're not going to be able to fill up an ita bag with merch for a relatively niche western franchise the way you can for flavor-of-the-season anime husbandos. There just isn't any official merch that's appropriate, and since selling fanmade merch is dubiously legal most artists won't post it in online stores like etsy or redbubble. If you're a really hardcore con-goer you might get lucky at artist alleys, but considering the fact that the Gorillaz fandom has been waning since 2018 odds of finding 2D pins and keychains are pretty low. Not that there are any conventions happening right now anyways.
I'd suggest just buying a button machine and making your own. It looks like that's what the person who made the bag in your pic did.