I have an ASUS P8P67 DELUXE motherboard, which (as far as I can tell) does not come with a TPM module, so I can't use BitLocker without a PIN or USB key.
Is there any way to buy and install a TPM module myself?
Some ASUS enthusiast motherboards do have a slot into which you can plug a TPM module (which ASUS sells separately). Unfortunately the P8P67 DELUXE is not one of them.
If you're merely trying to enable BitLocker on Windows 7 or Windows 8, you can do that by setting up a local group policy to allow BitLocker without a TPM.
It depends on if your motherboard can handle the installation of a TPM chip such as the AXXTPME5 by Intel. Or a TPM module using 20 pin headers, for example. You will need to check the specifics on your motherboard.
Portable TPM modules are, AFAIK, a thing of theory and not mass production. If your MoBo doesn't have support for it, then you're practically out of luck.