My tank bag is the one made by Bagster that came with the fancy tank cover and didn't come with a rain cover either. I went with a heavy duty trash compactor bag as a liner and stuff everything inside that. Not the coolest looking way to keep things dry but it works. Have been through many downpours and continuous rain showers with excellent results. I replace it every year just in case.
Bill W.

 

I have the DP tank bag and it came with a rain cover. I just keep it in one of the side pockets. It is a basic cover with a elastic to hold it on, but it works.