If you can't find Japanese cucumbers, look for a variety that has a firm texture and few seeds. A good choice is the Persian cucumber, but English cucumbers are also usable. These look similar to Japanese cucumbers and often come wrapped tightly in thin plastic. English cucumbers have more seeds than the Japanese version, and they work well in cucumber salads and sliced in tossed salads. The seeds are edible, but depending on the dish, you might want to remove them with a spoon.