You can't anymore, unless you already have a T-mob account with that data plan. That's kind of the point. Once you give up the grandfathered data plans, or "upgrade" to an Android data plan (at least $20), it's gone forever. So it behooves anyone who still has it to hold on to it.
Way way back, in the early days of wireless data, T-mobile had a $3? plan, on GPRS. Then it went up to $5. I have the $5.99. Then they started offering a $10 plan. That was available up until maybe last year? I think it's completely unavailable now.
Galaxy S4 T-Mobile
[ROM] Wicked v8
[Recovery] TWRP 184.108.40.206
Galaxy Nexus GSM
[ROM] XenonHD vXVIII
[Build] JZO54K 4.1.2
[Kernel] Leankernel 4.7exp2-180/interactive/deadline
[Recovery] TWRP 220.127.116.11
HP Touchpad 32gb
[ROM] Jcsullins 7/14/13 build with battery fix