I would call around to local bike shops, let them know what you are looking for and see if they can help you out. Our son is 5 and had the same problem with the small training wheels. He now has stabilizer wheels and toe clips that keep his foot from slipping. His bike is also geared down a little to make it easier to pedal. He only rides it around the end of our street and sidewalk and for short periods of time and loves it! If you would like I can post a picture.
i would love a picture