Ebay, either second hand auction sniping or a daily deal that you check out thoroughly with proper shops as a comparison.
I got a real bargain on an IBM thinkpad recently......full business machine (IE metal case so nice and sturdy, one of a couple of dozen sold by an outlet after some company upgraded their staff's laptops)
If you buy secondhand, the batteries are 90% likely to be fekked, but again, ebay will supply you with a new one cheaply....it's even worth checking the lists carefully to find one that is in good order with a dead battery, and looking up how much a new battery is to factor it in
Don't be tempted by the "i" fanboi's claims which I'm sure will come soon that a thinkpad will do the job for you.
It won't.
Nor will a BB playbook, or any other tablet........unless you have a desktop that you can synch up with and access via wifi and remote desktop
PS don't go for a netbook either......too slow, too limited and although temptingly cheap, you'll rapidly get very very frustrated with it.
A decent specced notebook or small laptop......if you want new, go get PC PRo magazine and see what's best recommendation, or even better, go to the dentist's and look at the copy on the coffee table says is best...cutting edge is 80% higher priced to start off with, you'll get a far better deal on a 4 month old 'just replaced at the top' type machine
Personally, as a second machine, I'd say go for a secondhand good spec well known brand
