We use "Puppy Pee Pads" or "Adult Incontinence Pads" and LOVE them.
These can be bought for far less at places like Wal-Mart, Target, TJ Max, even eBay has them for sale. They come in different sizes. We use 24"x36" on our larger Macaw cages and the smaller 21"x23" in the smaller cages.
What we like about them is the plastic backing on them and the absorbing material on top. The plastic backing doesn't allow any spilt water to pass through, thus, it saves your metal trays from prematurely rusting away.
Some will argue that the cost isn't worth it, but we have 14 cages and even then we feel the offset of cost is far worth it because it makes caring for that many birds a LOT easier. Tray change outs and cleaning are a lot easier with the use of them.
Please note, in ALL of our cages we use the bottom grate/grille to separate the waste tray and pads from the birds.