Around here, there are portable office building for sale on Craigslist from time to time. They usually are set up for an office with phone/internet wiring and maybe a bathroom. most are 10x30 or so, usually $2-3000.

I did a CL search, didn't find any locally right now but they do show up every so often.

Ebay or construction company auctions would be worth checking as well.

Do you have local zoning laws that might affect placement of a premanufactured building?