hi,
ultrasonic sensors are good but i'm not sure they are very much accurate at smaller distance. if the objective is to just find the obstacle, at closer distances, then ir sensors are good, also there are ready made sensors available. use two wheels to power the robo and third a dummy at hind to support them. you can place two sensors - one at front, one at any one side say right. once you get the obstacle chk both sensors, if right is clear, turn right, else turn left. for the above arrangement turning radius is a matter of concern though. hope i was of some help.
cheers,
neo