found one company that makes one, I am sure there are others,
Gooseneck Trailer Mfg. Co. - Custom Trailers, Frac Tanks & Truck Beds
I would think if one did a search and find companies that build utility and stock trailers they could build you one if you wanted, or even possibly they would build you a slide in unit, you would just have to contact the companies and see what they would do,
also do not over look local welding shops and fabricators, and or trailer shops,
(my guess is this is a one peice unit not a removable stock sides)
but plans for a build it your self, trailer
Trailer Blueprints — Livestock Tandem Axle Gooseneck Trailer | Trailer Blueprints | Northern Tool + Equipment
I really doubt if it would take much to make it a slide in unit like the above manufacture unit,
(one difference on a stock only trailer is usually the top of and the sides of the trailer are part of the support system, (usually no real frame or sides for support) but a utility trailer is self supporting (if one cut the top off a stock trailer one would not have much of a trailer, as some times the only "frame" a stock trailer is two angle iron on each side, where the springs attach to,
(so I would probably build a utility trailer and then add the stock panels on it),
(not built a stock trailer and cut or remove the stock panels top),
Trailer Blueprints — 16Ft. Tandem Lowboy Trailer | Trailer Blueprints | Northern Tool + Equipment
or
Trailer Blueprints — 26ft. x 8ft. 6in. Flat Deck Auto Hauler | Trailer Blueprints | Northern Tool + Equipment