Interesting question. I always thought that you had to win them in the slots, but after I read that Caesers had removed their S/S machines and were selling the coins at the cashier I checked it out and a few minutes later I was walking out the door $30.00 poorer with 3 more strikes for our collection!
On another note at almost all casinos that I ever tried they were more than helpful in swapping duplicates and even selling me the ones that I didn't have!
A slot attendant at HRH looked through an entire tray of $20.00 strikes for one as she refilled the machine. She even went so far as to switch plastic cases for me!
The only place that I can remember ever having any problem was Debbie Reynolds Casino....but that kinda speaks for itself now, doesn't it?