I'd grab some sort of T-Mobile prepaid SIM or MVNO (Simple Mobile, Walmart Family Mobile), as that would be the easiest bet. I think if you went the route of Straight Talk/Net10/Tracfone, you'd have to be absolutely sure it's a T-Mobile-based SIM and not an AT&T one.

This will work, too (despite being branded for the Fire) if your phone takes nano SIMs: https://www.amazon.com/Fire-HDX-8-9-Activation-T-Mobile/dp/B00L8BYGJQ/ref=sr_1_1?ie=UTF8&qid=1491002909&sr=8-1&keywords=T-mobile+fire+sim

The nice thing is you don't even need to activate it - just keep it around for unlocking purposes.