SIM card for Sprint Galaxy S4?

[Chicago Jerk]

I've always stopped at Digitel and purchased a SIM card for my phones. The kid at Best Buy is telling me that I have to have my Sprint phone unlocked in order to do so. I believe that he's full of beans. The phone has both LTE & CDMA networks in the settings. Can I do my usual SIM card purchase for around $20 at Digitel on Friday, or does some one have a "better idea"?
Thanks,
Bruce

OK... I just read that it's cheaper to buy a "throw away" phone at Hi-Lo of even Digicel. Any insight?