Everyone hands them out like candy - when I used to ask my TMO rep for "a few" (every phone comes with one, but sometimes someone breaks one), he'd come back with a pack of 100. No more. TMO usually charges $25. You can find them on eBay (TMO SIM cards - others won't work) for about $3-$4 shipped. Then you call TMO and give them the new card number and they'll reprovision the account.
(AT&T still seems to think the name of the game is "get rid of as many SIM cards as you can". The last time I had a problem, between the store in the front and the Device Support Center in the back, the phone must have gone through about 5 SIM cards [which can never be used again], before they told me that they were going to have to replace the phone. [Which they did, right there - and put yet another SIM into it.)
But for Asurion? Send them the phone. No case, no charger, no cable, no earphone, no SIM, no microUSB card, nothing. They strip everything out of the package before the phone goes into the shop. (Whether they steal it all or throw it all away, I can't say, but you're not the first one who's sent a phone with some accessory or other to Asurion, to report that it came back missing the accessory. And Asurion won't replace it.)