It's not a specialized tank bag, but The Waterproof Store
specializes in waterproof bags of various shapes and sizes. The 15L Dry Flat Bag
might work as a tank bag, in the sense that you could put it on your tank with some bungee netting type of stuff for a trip.
If you need a "real" tank bag that you leave on the bike all the time but can easily access things through a top loading pocket that's also waterproof, that's a bit tougher.
What I use for a similar need is a Mil-Std waterproof case that has locking latches that I can leave on the bike the whole time bungeed down to my pillion (I don't ride 2-up). Rather than a tank bag, I put it on the pillion seat. I got it for $40 plus $20 for the locking latches here
. It's great: I can fit an air compressor and tire plugging kit in there along with inline air pressure gauge, my EZ-Pass, mounting bracket and 12V charger for iPhone/GPS, zip ties, a small mint tin of quarters, my insurance and registration docs...
I put a cable thread lock
around it in case of theft, since I park it in the streets of NYC on a regular basis.
I could have gotten a larger (taller) version with more storage in it, but I wanted a lower profile case so I can still easily swing my leg around onto the bike. You can see it on the back of my bike in the picture I posted
of the Corbin seat I just got in the seat thread.