T-Mobile Website The regular price is $10. The 99¢ price is good through Thursday 2/6.
There are two types of 99¢ prepaid SIMs.
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. These can be used to activate a supported tablet on any plan including T-Mobile's 200 MB/month free plan.
I like to keep a spare T-Mobile SIM or two on hand to activate on the $30 100 minutes, 5 GB plan when I need lots of data while traveling. That plan is only available to new activations using a new SIM.
Related post: Prepaid Operator Profile: T-Mobile