I use a 2 1/2 gallon I bought from a ag store that's for spraying weed killer/fertilizer. Nothing special and is all plastic. It has worked alright for me and can be adjusted to a fine mist. I also just bought a 1 gallon ball shaped one. Kinda similar but the gallon on is kinda annoying, or both can be if you over fill them. That is if you fill them to the fill line, they have so little air space you have to pump them up often until the level go down and you have more air space.
I would say if you want to keep a gallon of product mixed up, buy a 1 1/2 gallon size. Two gallons mixed buy the 2 1/2 size ect.