Many people use the blue poly 55G drums for RO/DI and saltwater storage. Other than that, U.S. Plastics is a good source of containers or just the general knowledge contained in their catalogs.
One thing nice about some of the blue drums is they have removable lids so you can get inside to drill and plumb in bulkheads or uniseals and valves.
check with your local water company sometimes they install new water softeners and take out the old holing drums. The holding salt drums work great (55 gallon).