Where can I obtain a replacement TM6 departure card in Thailand after losing mine?

Here's the actual answer. :P

IF you are going to be LEAVING thailand you can as people suggested pick one up at the international airports.

HOWEVER

IF you're NOT going to be leaving and need the card for something immigration related (getting an extension, filing a 90 day report, etc) you need to get a police report saying you lost it and take that to your immigration office where they will look up your old TM6 number, give you a new TM6 card, cross out the number and write the old number on it>

They have them in the immigration hall.. just fill in a new one before going to the Immigration counter.