They are just laundry detergent jugs that are
sold in the supermarkets -- When I needed one, I just fished one out of the trash while I was doing laundry -- They have really good sealing if you use care cutting out the pour spout.
I have another one inside that I use at night -- I call it my Yellow Water Tank -- No hole in top of this one! I keep it under the front bunk with the porta-potty.
In morning when I go to the CG potty, be it outhouse or one with plumbing, I take both jugs and empty them.
BTW, I didn't generate much gray water -- I ate out of pots, used paper plates and licked my flatware clean! I also got a dish detergent squeeze bottle, rinsed it well, filled it with water and used it to apply water to the few things I washed -- Between that and the manual water pump, I used less fresh water and generated less gray water.