I got mine for free at the Apple store the day I bought my mini retina. I explain to them I want it to work on all the carrier and they told me, to be safe I should buy the Verizon version. Well the version that has a Verizon sim card in it.
They gave me a T-Mobile sim card for free so I wouldn't have to go to T-Mobile to ask for one. I do believe from what I've heard T-Mobile will charge you $10 for the nano sim card. I would try Apple first and see if you can get it for free.