Look for containers that are food grade for safely storing potable water. You can re-use a container that has previously help food goods like syrup, vinegar, and even ones that help peroxide. HDPE barrels will not leach chemicals into your RODI once cleaned out well unless they previously contained petroleum products.
Check out a livestock vet or livestock supply store for plastic barrels. Bottling companies also have them available from time to time.
If you don't go this route, many use the Brute garbage cans as they are NSF potable water safe.