I guess if you can get your hands on some copper pennies, that would work as well.
There are some "collectible" copper coins out there but you will probably get more value out of its rarity (from collectors) than it's weight.
If you ever find any pure copper coin in circulation, stashing them might be illegal (In some Countries, keeping an unreasonable number of coins is illegal, like the Philippines).
I don't think that it's rare. Copper bullion/bars/rounds can always be found on precious metal selling sites such as Apmex or Provident Metals. But upon doing some quick research, the best value bullion I could find was around $0.89 per ounce, while the spot price of copper was just over $0.1 per ounce.
Yes, there may be legal issues with hoarding copper. But as long as you don't melt them, it should be alright. But if you're saving up for a SHTF situation I don't think by that stage anyone would care if you melted them anyways.
I just want to get a head start in investing in copper, but I definitely need an efficient means to do so. Paying 800% over spot for the pure stuff is too much, while the legal concerns with copper coinage is deterring for sure.