You can get fleece longies on places like Etsy. Usually ones meant to cover a diaper as a cover will have a second layer - a contoured second layer for fleece shorties and an underwear-shaped second layer for longies (kind of like those skirt-and-undies-in-one things you often get with toddler girl clothes). The second layer helps to keep the wetness contained.

Also, they need to be 100% polyester fleece, no cotton in the mix or it doesn't work. Which you don't normally need to worry about if you're buying them as covers but if you want to makeup your own it's definitely something to check.

 

Ahhh ha, I see.

Well I just discovered making your own fleece covers out of cheap fleece blankets is super easy and cheap! No sewing, just cut the fleece, and use pins or cut it so u can Tie it on. I got a blanket from Walmart for 2.88, depending on how big I make them, I can get at least 5, probably more covers out of it. Just wanted to share ;-)

 

The fleece I have had wasn't hardly waterproof at all. Lanolized wool fleece would be perfect if you can find wool.