@Jack ****
Hmm, I'd say IF you got a police report stating you "lost" the TM6 departure card, you could use the immigration office in the area where you are..
They'd give you a new TM6 (looking up the old one, crossing out the new numbers and writing the old number on it).
OR
you could try it without a police report but most places ask for one and seeing as they're free at any police station I'd get one just to have it when you go.
If you do get the extension down there remember that an under consideration stamp has to be closed out at the same immigration office that issued it (SO if you get a 14 day under consideration stamp you'd need to hang tight there for 2 weeks before you go back)