Waterproof blanket liner?

I saw this advertised in a magazine and forgot to write it down! It's a waterproof blanket liner you can put underneath of a fly/open-weave sheet so you can have a fairly lightweight rain sheet... has anyone heard of these? Where do you get them?
It's too hot out for Maia to be in a full rain sheet, but yet it's raining on and off so I'd like her to have some sort of protection... I have a textilene blanket (it's made out of the same material as some lawn chairs) and so it's fairly open-weave, but I LOVE it. It fits well and keeps bugs off, but at the same time is very breathable, and very long-lasting, it takes a LOT for a horse to rip through!
http://greenhawk.net/cgi-local/SoftC...b43+1214392184
^ this is the blanket I have.
The sheet I'm looking for is fairly small, you would think it doesn't do much, but is waterproof. The one I saw was white. Any help would be greatly appreciated!

Is this what you're looking for....
http://horseware.com/usa/flyprotecti...sheetliner.asp

This is a waterproof fly sheet liner that is made by rambo.

Perfect!! That's exactly the one I saw in the ad, thank you so much mudypony!

Thats really neat, I've never heard about it until now.

Your welcome JDI. Glad I could help!