AGP is a type of interface. It stands for Advance Graphics Port. There are four types of main graphics interfaces, PCI, AGP, Onboard, and ISA. ISA is an obsolete interface and used only in antiquated system replacement models. Onboard, which is interconnected directly to the motherboard, is a non upgradeable interface. The last two interfaces are still regularly used, but AGP is becoming more and more obsolete by the day. By technology standards, it's End of Life was passed almost three years ago when the PCI interface had a new and better revision released called PCIe or PCI Express. The PCI Express interface is the most common interface used in computing today. The latest revision of PCI Express called PCIe 2.0 allows for much faster data transfer rates than PCI and double of that of it's most recent revision of PCIe X16.
Clear as mud?!?! 
Finding AGP video cards can still be pretty easy. Try using NewEgg or Tiger Direct. Make sure you select the proper interface in your search.
Note: While video memory is very important, data transfer rates are more important on video cards. When looking through your results for AGP video cards, make sure to take note of what the memory value is and what density the interface is. So, if you find two results that read as follows:
XXXX brand AGP video card, 512 MB Memory, 256 bit Interface
XXXX brand AGP video card, 512 MB Memory, 64 bit Interface
You should select the first one because of the higher interface. Good luck and reply back with any further questions!
