The venerable microSD card slot is not as common as it once was, but there are still phones that let you cram in a little more storage. You just need the card, and Lexar's versions are on mega-sale through Amazon's current Gold Box deal. You can get as much as 50% off a 16-64GB class 10 microSD if you act now.
This is a sale on a variety of Lexar flash memory products, but the microSD cards are most relevant to Android devices. On the down-low, there are also flash drives and full size SD cards included in the sale, but you didn't hear it from me. The microSD cards come in two speeds—300x and 633x. The slower ones are on sale for as much as half the cost of the faster versions, for whatever that's worth.
You can grab a 64GB 300x card for $30, which is 33% off. The 633x 64GB card is $50—47% off. Just need a little storage? The 16GB card is $7.49. All the cards appear to have Prime shipping as well.