T-Mobile Website now through 4/03, while supplies last. The regular price is $10. The kits are available in mini (aka regular), mini/micro combo and nano sizes.
The SIM kits consist of a SIM and an activation code and can be used to activate a phone on any T-Mobile prepaid plan including the 5 GB of data for $30 plan, which is only available with new activations.
By themselves, without the activation code, these SIMs can also be used 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