Amazon lists the 4.0 CDs with cable for around $5.
It doesn't matter what version you get, as long as you have a high speed
internet connection.  They will all upgrade to the latest.  Some of the
upgrades are several MB.  The older versions are $2.

You can also download directly from the vendor, but that's more expensive.
http://www.bvrp.com/customers/motorola/upgrade/US/
It says it is an upgrade, but it is a full release.
If you wander through the forums, there is a 15% discount code posted.