Is there an effective way to get men's shirt makers to produce bigger size shirts?

I need a 4xl shirt. I’m a big, tall guy. I wish I weren’t but that’s how it is.

I get ‘targeted’ advertising on Facebook and other places for shirts – but, depending on the distributor, their largest size is either 2x or 3x.

How can I get these companies to understand that they have a willing clientele if the only made the right size?