T-Mobile Website has Prepaid SIM Activation Kits on sale for a penny with free shipping if you use code SIMDEAL. The sale runs now through 5/19, while supplies last. The regular price is $10. The kits are available in mini (aka regular), mini/micro combo and nano sizes.
The kits consist of a SIM and an activation code. The activation code is required when activating a new line of service on any T-Mobile prepaid plan including the $30/month 5 GB of data, unlimited messaging and 100 minutes plan, which is only available with new online activations.
By themselves, without the activation code, these SIMs can also be used as replacement SIMs for prepaid or postpaid accounts or to activate a supported tablet on any T-Mobile mobile broadband plan including the 200 MB/month free plan.
Related post: Prepaid Operator Profile: T-Mobile