Wow, If you are like me finding a used modular office building was time consuming so I found http://www.imodular.com/ and used their service. Just go to the home page and click on one of the very obvious free price quote links.

Once I completed their request form I received calls from local suppliers with used construction trailers and modular buildings for great prices. They all competed for my business and I didn't buy the least expensive but I did get a fair deal. They answered all of my questions too.

Here's the modlar office I rented for my construction site in Florida.
24'x40' Used Construction Trailer