Sadly older episodes are hard to come by, BBC Store have most of the old christmas episode and a few others.
They do exist if you know where your looking. They have just released a variety of EastEnders episodes on Amazon but they cost money to watch. There are also a variety of private torrent sites (sadly, you need invites) I would post them on here but don't want to risk it.
Give me your email (if you have one) and I'll see what I can do for you :)