“Hi, I’m looking for an inexpensive Memory Card for my Galaxy A21”.
Ok, easy enough. The Galaxy A21 uses a standard MicroSD Card, you can get those in different sizes so any of these below will work for you:
SanDisk 128GB Ultra MicroSDXC UHS-I Memory Card with Adapter – 120MB/s, C10, U1, Full HD, A1, Micro SD Card – SDSQUA4-128G-GN6MA
(Link: Amazon) $19.99
SAMSUNG: EVO Select 128GB MicroSDXC UHS-I U3 100MB/s Full HD & 4K UHD Memory Card with Adapter (MB-ME128HA)
(Link: Amazon) $19.99
Both of the options above have larger sizes, 128GB is probably good for most users needs however you can choose larger if you feel you need more space for photos, music, apps or etc …