28 oz mug for sublimation?

That size isn't as popular as its 11oz. and 15oz. sizes, so you'll most likely have to go to Alibaba to get wholesale prices. There are several mfg's that sell that size but get ready to purchase at least 3,000 to get a decent price.

If you do find anyone in the U.S., it most likely won't be wholesale, but maybe you can strike a deal to pay a little over wholesale for 1xx amount of mugs.

All the sublimation stuff has a flat surface - well sort of, aside from the standard low grade manufactured stuff from that famous source country. Those mugs are actually slightly tapered and there are numerous hills and hollows across the surface.

The mug in your photo has a compound curved surface. That means, the curvature is present in several planes.

Flat transfer paper and conical mug presses won't fit that contour. You need something like a 3D vacuum press to do that. And then you need a way of holding the flexible transfer in constant contact with the mug surface. If the press does not have sufficient vacuum, there is a sort of stretchy wrap which looks like a tensor bandage which may work.

As for the regular conical mug presses, they do not have a cuff compatible with anything larger than the 15 oz. mugs.

A latte mug cuff won't work either since that doesn't accommodate a compound curve - only a flat surface.