Not only is there “lots” of water in the oceans, as the water spills out of the canal (whether into the Pacific or the Gulf), it replenishes the supply. The canal flows down from its apex at Gatun Lake in both directions. There is, however, the problem of pumping the water to the apex of the canal. Possibly also the problem of mixing fresh water and salt water in the various lakes that are part of the canal.
As many have commented here about pumps and pumping. There are no pumps used to raise ships in the locks. This is pointed out in every detailed description of the engineering design.
Gatun Lake is higher than both ocean levels. Water raised in the lower locks is or has been up until now— drained out into the oceans. Pumping these quantities of water would be quite expensive in energy to pump.