Frankly outside of the VR|46 brand, I think the rest of the riders' swag is garbage unless it's coming directly from the a marketing heavy manufacturer like KTM.
Try finding decent Aprilia swag or a tshirt from any of the Satellite MotoGP/Moto2 teams and you'll run into the same issue.
Most of these folks use the same vendor like Top-Racers... hahaha which I just checked and they do have Joe Roberts gear!
https://www.top-racers.com/us_en/riders/joe-roberts.html
I'll finish my rant anyways, biggest problem is that all swag/chotckies vendors struggle to make money with the volumes of a single riders which is why there is only a handful of vendors that carry several riders just for economies of scale... Unfortunately, unless an owner of a promo company becomes a MotoGP fan and decides to run a store just to appeal to nutheads like me without actually making money I don't see this changing any time soon.
BTW - a JR tshirt with shipping is $54 yowza...