Most Walmart locations stock "Bring Your Own Phone" (BYOP) kits that include 3-in-1 SIM cards (standard, micro, and nano sizes): Sim Cards - Walmart
Yes, you can buy a SIM card at both in-store and online . They carry a wide variety of prepaid SIM kits from major and regional carriers, typically starting at around $4.88 . Common Carrier Options at Walmart