I've put together some links to bbq smoker trailer manufacturer's below. I don't own any of these units listed and don't endorse a particular manufacturer over another one, but I thought it might be nice to list links to all these manufacturer's in one place.
Cookers and grills This manufacturer from Union Point, Georgia has designed some nice looking smoker trailers that incorporates reverse flow design.
Lang I cooked with a Lang Model 84 several times with another competition team when I was just getting started. It's a popular offset for a reasonable price and it will cook large quantities of bbq. They're now building a twin Model 108 for those that need to cook massive amounts of 'que.
Peoria Custom Cookers They make them big, small and every size in between. From backyard to professional, they've got a cooker for almost every cook. Take a look at their Meat Monster cooker. It packs a lot of nice features in a small package.