The list of providers with unlimited data plans is extremely limited. Basically, you're looking at Spring for postpaid contract service, and then Sprint MVNOs like Boost or Virgin for prepaid service. The Ally may be flashable, but I have no idea. A better place to look might be a forum on flashing.
You might look into PagePlus. No flashing required. No unlimited data either, but very competitive rates for voice, text, and OK rates for data.