T-Mobile Prepaid Site has put Prepaid SIM activation kits and Prepaid Mobile Broadband SIMs on sale for 99¢ each. All sizes are available; mini (aka regular), micro and nano. T-Mobile normally charges $10 for SIMs and activation kits.
The SIM Activation kits contain a SIM and an activation code and can be used to activate a phone on any T-Mobile prepaid plan. The Broadband SIMs are for Wireless Hotspots, Laptops, Tablets and USB Modems.
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.
The sale is good through Wednesday Sept 18, while supplies last.