If you buy 3 or more OEM licenses from an online retailer, you may just get the OEM Preinstall kit CD thrown in. A standard Microsoft System Builder kit is a small cardboard box just big enough to hold 3 shrink-wrapped OEM CDs, stickers, and manuals, and in EVERY box the OEM Preinstall CD is just thrown in for your convenience.
The 3 or more is the key.. how many of you have bought an OEM license via NewEgg or some such? Then how many of you wanted like 3 or 4 OEM license CDs? I'll bet very few people have ever ordered more than one copy, so your chances of getting the preinstall kit are much less.
So, basically for a source, you should look to NewEgg or other nerd-oriented parts stores that will sell OEM licenses with some other inexpensive bits of hardware.
-Scalar