Yeah certified food grade IBC's are the ones to look for - but it pays to ensure and spell out that it's going to be storing potable water and can't have contaminants. If the seller can't evidence that the IBC is clean don't buy it. That's even for storing garden water...
If there was any doubt about them and you can't find one locally that's clean you can buy them brand new and they aren't that expensive for the piece of mind factor. You can also get them that the bladder or tank is not opaque so you have less of an issue with algae and stuff growing in the tank.
