Unfortunatly, there is no SIM card on the Sprint iPhone 4s that is pre-loaded.
Sprint and Verizon uses a CDMA network, which does not use a sim card, but it is programmed in directly into the phone. While there is a SIM card slot on the iPhone 4s, (since it is a world phone), you may not be able to use it, since the device is carrier locked.
However, if you have good account standing, you can call Sprint, and they may be able to temporarily unlock your phone if you plan to use it overseas for vacation. My friend had similar situation with Verizon iPhone 4s, but since his account was in good standing, they unlocked his phone for two weeks while he was on holiday in UK.
(please do not attempt the Unlock method using various tool online, since it is now illegal under DMCA, and you could risk some prision time)
As for a sim card, the pin needle has to be specific size. If it's a very small pin needle, it's not going to work. It needs to be a pin needle similar to one used to thread strings in quilting. Yes, I had similar issues when trying to open my iPhone 4 SIM card after I had it unlocked by at&t so I can swap it with new at&t sim card under different account. (and boy, it was frustrating, until I decided to use my mum's thread needle). If you are willing to spend some money, try eBay for SIM card ejection tool. (Google works too)