Typically toter type boxes are generally not removable to go from truck to truck like a standard sleeper. They are built in place and have fairly complex systems that tie into and around the chassis. So it is build it yourself or have it built. Some folks on this site have had very nice projects starting with a "box truck" box and building out the interior from there, so that is an option. Also, if you have deep pockets ICT builds traditional type sleepers, with all the comforts of a toter conversion. Of course everyone's first advice that has built their own (including me) is go look on
Race, performance & street cars, engines, engine parts, trailers for sale and try to buy one already done that will meet your needs. You will spend far less that way than building yourself. Unless you just want a hobby. I think Kenn mentioned in a post this week that he was 4 years into his build, and still has work to do.