SDRAM structure for Cortex-M7
The type of memory you need is SDRAM. It is not the same as DDR SDRAM. I don't think I've seen a Cortex-M device that supports DDR memory yet. You would typically go for Cortex-A family, if you need that kind of capability.

You can easily find SDRAM chips on digikey or any other distributor site. 

At this point, the rest of you question is probably irrelevant, but let me give you some extra points.

Both of the chips support 256MB of RAM. If you look at the digikey link above, the largest amount in a single chip is 512 Mbits (64 MBytes). This is where the data bus width comes into play. You can see that 512Mb chips come in three configurations:


16M x 32bit
32M x 16bit
64M x 8bit


One way to increase the memory capacity would be to use two 512Mb devices in 32M x 16bit configuration, and wire them up as shown in this Atmel app note, for a total of 128MB of RAM: