Last year, I could not find any box on freecycle when I needed to send some stuff back home and finally I had to buy them. Local business Rocket Van, near East Street market sell them, they were not so expensive (well depends on how many you need). They have a shop there where you can see the selection they have but they also deliver for free I think.
Here's a tip from inside the industry:
Hunnex boxes, just south of Burgess park in Parkhouse Street.
Tell them Graham from Bigvan sent you.
I was there earlier today, got a load of strong boxes that seal easily, and can be stacked to save space in the van - it's probably worth investing in a couple of rolls of sticky tape too.
If you value your stuff protect it - supermarket biscuit boxes filled with books can tear open, and can be too heavy to lift and carry.
Sheila at Hunnex has years of experience, and is very helpful.