Q: Where can I buy more RAM? And is it hard to install?

You can buy RAM in Internet and install it yourself except in the Late 2012 21'5-inch iMac, where you will void the warranty if you add more memory.

You can buy more memory at OWC or Crucial. See > http://support.apple.com/kb/HT1423?viewlocale=en_US

OWC (macsales.com) has excellent how to install videos on their site:

http://eshop.macsales.com/installvideos/

And it's quite easy - jsut make sure you push it in firm enough so it is seated properly.

Which iMac do you have? More memory than needed won't hurt to the computer

1 GB is the minimum RAM for Mac OS X Snow Leopard, but I suggest you to put more memory, so Mac OS X Snow Leopard will work faster > http://eshop.macsales.com/shop/apple/memory/iMac/Intel_Core_Duo

An Early 2006 iMac supports 2 GB, so I recommend you to buy 2 GB of memory

You are welcome