I see your problem there, honestly I don't have the answer to that question. Maybe for now so you don't take the risk you could find something else to mix the copper in? An old milk jug even, just fill that up and mix copper in a gallon at a time? I know it's painstaking but it'll at least tide you over until you could pick up another 5 gallon bucket or something.
Honestly I have no idea on that, I think most people just pick up the standard buckets but I could be wrong. I remember lowes had a food grade one that had gallons marked on it and I regret not getting that one over the standard blue one for ease of mixing. Sorry I can't be of more help, but for what it's worth I rarely see people worrying about food grade unless it's a sort of long term water storage thing, but even then a lot of people use Brute trash cans so I really don't think it matters too much but if you're hesitant I don't think it could hurt to get food grade ones.
