Typically on this engine if it is leaking this much oil from under the timing belt cover when the engine is running it is the seal on the oil pump drive shaft is leaking. This looks like a typically round lip seal with a metal housing.
There is a preformed o'ring that goes between the oil pump body and a large housing that bolts to the front (timing cover side) of the engine. This o'ring could look like a fish.
Have yet to find this molded o'ring any place but a Toyota dealer (a few dollars). Tell the Toyota parts person this is premolded o'ring that goes between the oil pump and the housing it bolts to. Its about 3 x 4 inches more or less, not that large.
The cam and crankshaft also have a round lip seal where they come out of the engine. Often owners also replace these seals as the most of the parts are already removed to change the timing belt.
I doubt if the preformed O'ring would leak, but never know.
Once the timing belt is removed the oil pump can be unbolted and the o'ring replaced.
If it is the round lip seal on the oil pump drive shaft it can be replaced by removing the drive pulley on the shaft then pulling out the shaft from the housing. A straight forward job when the pump is off the car. The pump has 2 gears, a drive and a driven. If taking the pump apart make sure a dot on the face of each gear faces the same direction.
Thus if you want replace all the seals there is:
A lip seal on the camshaft and crankshaft.
Oil pump drive shaft lip seal.
Premolded o'ring between the pump body and housing bolted to engine.
If you remove the tire and start the engine (make sure the car is blocked up well). If the oil pump is leaking, oil will start dripping midway from the timing cover on the firewall side.
The other seals can be purchased from 3rd party sources although the dealer should have them.