Most if not all R/C Transmitters are for Aircraft and very few R/C aircraft have a sound chip which means that not many Transmitters have volume controls,
Your Tank will have a sound chip so finding a Transmitter with at least one rotary or slide control is vital,
Turnigy and Flysky Transmitters do have rotary controls and are available with 9 channels,
My own favourite is the Turnigy TGY9X but it is no longer available but the FLYSKY 9 is the exact same model with a different name and the receiver from one Transmitter works fine with the other,
Turnigy 9xrpro is the new Transmitter and comes with a good reputation.
Shop around for a good price.
Shaun.