The last several years have brought larger internal storage to smartphones. It's now rare for a high-end device to ship with less than 64GB of storage, but even upgraded models with 128 or 256GB might not be enough for your needs. That's when a beefy microSD card can come in handy, but the largest ones cost a pretty penny. There's a sale on one today, though.
You can snag SanDisk's largest card right now for a sizable discount, though. The 400GB card is just $185, down from $250. The price has been sticking close to MSRP since the card became available, and this is the cheapest it's ever been. The 400GB card is rated UHS-I with transfer speeds of up to 100MB/s. It's also an A1 card, which SanDisk says is better for application performance.
The card is on sale for the same $184.99 price at both Amazon and B&H. However, Amazon says it'll take 1-3 weeks to ship. B&H has it ready for immediate shipping. B&H is probably the way to go unless you live in New York or New Jersey where B&H has to collect tax. Shipping is free on B&H and Amazon.