How can I increase memory for my MacBook Pro?

It seems that my MacBook Pro has two slots of Memory that currently have a 2GB in each (Total 4GB) that can be upgraded to 4GB each (total 8GB). However, when I try to go to the Apple Store to purchase then, it says that they are unavailable for my model of MBP, even though I am looking up the correct model (MBP, early 2009). Not sure if this means that I can't upgrade the Memory OR that the 4GB 'cards' are not available in the Australian store.

Would love some info on this, thank you! 🙂