T-Mobile Website. This time around you need to enter the prom code NOVSIM at check-out to make them free. The deal is supposed to be good today 11/11 and tomorrow 11/12 only.
There are two types of SIMs that are free with code NOVSIM:
Prepaid SIM Activation Kits which are for phones only. They contain a SIM and an activation code and can be used to activate a phone on any T-Mobile prepaid plan. SIM Activation Kits are available in mini (aka regular), micro and nano sizes.
Prepaid Mobile Broadband SIMs work in tablets and mobile broadband devices only. They are also available in mini, micro and nano sizes.
T-Mobile normally charges $10 for SIMs and activation kits.
Even if you don't need a SIM right now it's good to have a spare handy as SIMs do go south sometimes. An active or inactive T-Mobile SIM can also be used to get the data settings menu to appear when setting up an iPhone on AT&T MVNOs.