What should one do if he had intercourse with his wife before Fajr and did not find enough water for ghusl for prayer?

If the water available to you is not sufficient for ghusl, you must search for enough water. If you do not find it, use the available water to wash some parts of your body from janabah, then perform tayammum for the remaining parts of the body. This is the most cautious approach. Some jurists have gone to the extent of saying not to use the water if it is not sufficient for the entire ghusl, and to suffice with tayammum. Others said to use it without performing tayammum, but the first opinion is the strongest and most cautious.