Actally, there are plenty of man-made materials in the city where you are. A brick, a concrete block (improvised window shopping device), a sidwalk. All of the previous suggestions are good. Depending on how dull the blade is, glass can work very well. Glass and ceramic are harder than steel. Personally, I improvised by using a woodworking upright belt sander, removing some junk from it and I had a professional grade sharpener. But I seldom venture far from the extension cord or the beer frig.