A look at what is currently available on eBay
 MAINSTAYS WATERPROOF MATTRESS PAD PROTECTOR QUEEN US $3.49 
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 Beautyrest Top Waterproof Crib Mattress Pad B277CN-CRIB US $22.00
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 Baby Crib Quilted Mattress Pad Stain Cover Waterproof US $29.00
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 200TC WATERPROOF POLYGEL MATTRESS PAD CAL KING SIZE NEW US $28.99
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 200TC WATERPROOF POLYGEL MATTRESS PAD FULL SIZE NEW HOT US $23.99
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 200TC WATERPROOF POLYGEL MATTRESS PAD QUEEN SIZE NEW US $25.99
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 CAL-KING WATERPROOF MATTRESS PAD COVER US $127.88
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 MATTRESS PAD KING PREMIUM WATERPROOF ULTRA ABSORBENT US $31.99
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 PREMIUM FULL ULTRA ABSORBENT WATERPROOF MATTRESS PAD US $20.99
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Mattress pad?
Do I really need a waterproof mattress bad for the baby's mattress? Our mattress has a waterproof covering and we put a sheet over it. I figure if the diaper leaks, the sheet will soak it up and then we can clean it/wipe it down. What's your opinion?
Sounds like you have a good set up. 
The sheet will soak up most of it, and just toss the matress pad/covering in the wash once a week if you have a bunch of leaks to keep it from smelling.
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