T-Mobile Website. The regular price is $10. The deal is good from today 1/13 until 11:59 PM PT Thursday 1/16.
The SIM kits 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.
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.
Related post: Prepaid Operator Profile: T-Mobile