The girls from mllsdemode did a blogpost about the wonderful item that is a bag in bag.
I bought mine at amazon.co.uk but there are plenty of webshops selling these or variations on these bags.
These bags are heaven sent when you often change handbags...of a considerable size. ;-) I like to swap handbags for work now and again and like this I'm sure to always have my stash with me.
Very happy with this purchase!