ESF_riverside and esf_lavapit are the default maps with senzu beans.
In riverside, go under the water and slowly move through the water. You may or may not see small pieces of darkbrown sticking out of the sand. You'll know you have them when you see your senzu bean count increase. They're hard to see, so if somebodies tryin to pound your ass, just fly through at top speed.
In lavapit, you'll never see them. Stand on one of the fire catawalks stretching from the main area to one of the 4 small areas. Look toreward the center, you should see a corner, and a white light. Go torewards that light, and step in it. You should now have 3 beans. There are 3 more bags on the other corners aswell.