We bought an IBC tote off of Craigslist for $30. It had been used by a homeowner for storing extra pool water (chlorinated). We rinsed and aired it out for about a month before we set up our small catchment system.
I'd suggest Craigslist or Ebay
local delivery. There are probably better availability options elsewhere, it's just that we had success with that.
Interested to hear others' ideas, as we'd like to expand our water storage tanks as well.