Industrial Paints and Plastics is good but it can be pretty expensive too.

You can get the 5 gal water jugs at any place that sells ro water..safe way, save-on etc...

I've been using a plastic Rubbermaid 'Roughneck' rectangular storage container for years. Comes with a hinged lid & holds about 23 gallons comfortably. Not even sure if it's considered food safe but haven't had any issues with it in that regard. Drilled a small opening near the top, added a JG bulkhead & the RODI feeds directly into it. I also mix up fresh saltwater in it for water changes. Although I do have a number of the food safe white & blue drums shown in this thread, I use most of them as rain barrels. Also use one to collect RODI waste water for various uses around the house & garden rather than sending all that water down the drain. The city charges quite a bit for water around here these days & at the ratio of 1 to 4 good to waste that most RO membranes work on, the waste water adds up in a hurry.

An alternative I haven't seen mentioned here, is that any spare aquarium tank you have kicking around would make an ideal water storage vessel.

I went to Johnson plastics and picked up two thirty five gallon barrels( in Edmonton).

I use a grey Rubbermaid trash bin as well.vi think it's 45 gal. It had a sticker on it when I bought it saying the city of Edmonton would not empty them as they are too big. I have my rodi plumbed to a mechanical float used for cattle waterers from ufa. Works well and holds around 35gal.