You don't need the plan. Don't try to buy the plan then cancel etc. the SIM cards are readily available at discounts without plans attached. You can buy a pre-paid SIM, a data a SIM or a phone SIM. All tmobile sims are the same and can be used for any purpose. Goto slickdeals.net and search for tmobile SIM deals and you will see they regularly go on sale for one cent and sometimes for free. It is strange to enter a credit card for a zero dollar sale, but you may have to. The other alternative is to go to a drug store or discount store and buy a tmobile sim kit, they are usually $10. Try Walgreens, Walmart, target, etc. DO NOT BUY A SIM WITH A PLAN! Look harder and you will find a way to do it.
Here you go I did it for you. Tmobile prepaid nanoSIM for an iPad Mini w/retina for $0.99 with coupon code "SIMDEAL99"
http://prepaid-phones.t-mobile.com/...=1225267&SID=963e17d560ef4b89b6f1dd26ba53e204
It doesn't matter if it is prepaid. The SIM will work for any tmobile pre/post/phone/tablet plan, including the 200meg/month free plan.