Hi, and welcome to the Bulletin Board!
At the beginning of a cook, with the lid and cooking grates removed, I just pour water into the pan from above using a dedicated 1 gallon gasoline jug. In the middle of a cooking session, I replenish the water every 2-3 hours through the access door. The long plastic spout on the gasoline jug makes it easy to fill the pan this way.
In the past, I've used a 1 gallon milk jug with good results, even through the access door. Others use garden watering cans with long spouts, or 2 litre soda bottles. You can even use a garden hose!
Regards,
Chris