Purchasing online often provides the widest selection, especially for specific network requirements: Go to product viewer dialog for this item.
Physical stores are the best option if you need a card immediately. You can typically find them in the electronics or prepaid mobile sections:
Micro SIM cards are readily available through major big-box retailers, official carrier stores, and various online marketplaces. While most modern smartphones use the smaller Nano SIM, many retailers sell that include a Micro SIM size that can be punched out of a larger card. Where to Buy Locally
2 Pack T-Mobile 4G LTE Sim Card New Unactive All Size 3-in-1 Regular Nano Micro