Like Rod noted, I have seen other modelers use regular manual laborers, such as track workers, and turn them into prisoners by painting their clothing to resemble prison uniforms. Color(s) would depend on the location and era. Pre-1940s would be stripes and chained together while working on a road or other construction project. Present day could be blaze orange and picking up litter, or doing basic grounds maintenance in a park or at a rest stop.
You will most likely have some painting and detailing involved. I don't know of any "ready to serve time" prison figures being offered by the manufacturers. You could try to make a plea with Just Plain Folks Dad. He reads postings here, and sometimes posts things himself.
Best of luck,
David Meashey