S-scale is blessed with a large selection of human figures, readily available. Most will require some painting, but there are some pre-painted ones available.
On average, male humans are between 5'4" and 6'4" tall. Female humans are typically between 4'10" and 5'10". In S-scale that translates to male figures being between 1" (25.4mm) and 1-3/16" (30.1mm) actual heights, and female figures being between 29/32" (22.9mm) and 1-3/32" (27.8mm).
Model figures are sometimes referred to by their average physical height in terms of millimeters, especially European-sourced ones and/or military figures. Typical sizes, at least applicable to S-scale, are "25mm" (5'4") and "28mm" (5'10"). So, figures so identified are potential candidates for use in S-scale.
When looking for S-scale figures online, be aware that some are flagged as S-scale or "1:64" with the intended audience being the matchbox-style collector, but these figures are probably closer sized to HO-scale than S-scale. Conversely, figures that were made in the A.C. Gilbert American Flyer era, or targeted at those modeler tend to be oversized. If scale-accuracy is important to you, spend some time doing research, ask other S-scale modelers, or buy one figure to test out that manufacturer's product line. The S Scale Resource magazine article linked to below offers a good overall review of the figures on the market today.