Franklin T9

https://prepaid.t-mobile.com/prepaid-internet

Any prepaid SIM should be fine.  It.is the plan the SIM is setup with, which you can see what is available at T-Mobile's website.  If you go through selecting a plan and setting everything up, it should give you the option to order a SIM as well.

Thank you, but I don't mean the size.

I want to buy (online) a prepaid data sim card that's compatible with the Franklin T9 but I can't figure out which one will work ...

SIM cards can be popped out in 3 sizes.  Each SIM comes in this pop out format.  The SIM card size you need to pop out is nano-SIM.

https://www.t-mobile.com/support/tutorials/device/franklin/t9-hotspot/topic/battery-memory-amp-storage/insert-sim-card/3