Just curious where people get their mailing boxes from?

I'm looking to get a better price than Australia Post who charge several dollars per box and don't discount a great deal from that price.

Officeworks seem to be the same kind of price.

Signet.net.au appear to have them at a reasonable price but when you factor shipping its not that great of a deal.

Are there any other suppliers I should know about, who'll do a bulk price on 25-100 units?