Interbank means the rate traded between banks - it's not a place as such that customers like us can access. It's useful for knowing where the real price of the currency is though to assess how a good a deal on offer is.
Generally travel agents/bureaux de change rates are a few cents below the interbank rate (if you're buying that is, they are well above if you're selling), other methods like Fairfx and the like have more competitive rates.
Then there's good old Crown Currency whose rates seem to come from somewhere I don't understand