We always just used the tub in the house.

I wonder if you could buy a double sink and cut down most of the partition. It would likely compromise the structure somewhat, so install a 2x4 support frame underneath. Just a thought, anyway.

We do use the bath tub now but in the new house my wife says it is off limits for the dogs. She wants the laundry tub to be in the garage for the dog washing. She volunteers to be the official dog washer.

I will have to look into the modified double sink resolution. I was up at Lowe's tonight looking around but didn't find anything that would work.

You want to try something like Farm Fleet. Farm supply stores carry stock feeding and watering items, I bet you could find something suitable there.

Careful with the double sink option. Make sure the basins are not separated near the bottom or it will limit how far down you can cut the partition. HD and Lowe's have double's in stock, but sometimes they're on the upper shelves somewhere.

Cheap alternatives:
* largest storage container you can find, and install your own drain
* mortar mixing tub

Uhh, how about a tub?

You can buy a cheap one ($109 @ lowes) add a drain kit.
Build a platform if you want to raise it up a bit.

Yes, a smaller bathtub would be my suggestion, or an old one. If it doesn't have to be pretty you could get one at the ReStore. I'd recommend a platform too. People remodeling bathrooms will also sometimes offer old tubs for free on Freecycle, Craigslist etc.

I've also seen people use shower bases for dogs, you could make a mini raised shower with short walls around it, but if you want something you can fill, I'd use a bathtub.