|NVRAM stand for Non-volatile RAM and no, you wont find any on sale because your computer already has all the NVRAM it will ever need.|
NVRAM is also know as CMOS RAM. It is a the small amount of memory usually around 256 Kbts, (thats kilo bytes, not mega bytes) that the system stores it's setting in like your disk configuration, video, how much ram you have etc. This information is read by the BIOS at boot up and is never used again.
It is called NVRAM because it retains its setting when the computer is turned of unlike normal RAM thats loses all its contents when power is removed. Power is maintained by a small NiCad battery on the motherboard - also called the CMOS battery. This usually last about five years.
CMOS stands for Complementary Metal Oxide Semiconductor. It is a type of RAM that has very low power requirements, but is relatively slow compared to SDRAM. As it is only read once at boot up and there is not much of it, this is not a problem
Your computers main memory will be one of the others you mention, if it is a relatively new motherboard, most likely DDRAM.