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2012 Canadian Penny... then in 2013 they are scarce!
Canadian pennies prior to 1997 were approximately 98% copper and 1.75 % zinc. So, a pre-1997 Canadian penny (aside from being illegal to smash in Canada) should give you a decent result after elongating. Note: Do not attempt to elongate a "Loonie" (the non-round coin). It will squish far too short and you will loose part of the design.
Mini M Containers work great for storing your quarters and pennies when going on a squishin mission. The containers holds exactly 40 quarters and 20 pennies -- enough to roll 5 sets of 4. You can prestack them (2 quarters and penny, 2 quarters and a penny, etc.) then just slide the number of coins you want to use out in your hand and youre ready to go.