Running case fans on D cell batteries
The Fans you have selected are quite reasonable: you can find more data on them here. 

Arctic characterize the fans this way:

 


For your battery I'd suggest you use a sealed 12V 10AH motorcycle Lead Acid type battery instead of buying NiMH 1.2V D cells. While the bundle your point to has a charger, it's mains powered. Connecting this type of NimH batteries to your car system is complicated and you need a specialized charger. With a Lead acid battery you can simply charge the battery from your cigarette lighter or other utility connection and not have any problems. If you are worried about charge current (and someone is sure to bring it up) you could simply add a series 0.2 Ohm series resistor to ensure the charge current never gets too high. Plugged into your cigarette lighter or utility connector you will be able to operate the fans and charge the battery at the same time.
Since you seem to be using Amazon as your source here is the type of battery I suggest. 
There are lots of options, but your should be able to get a battery in the $20-30 range that adequately meets your needs.


Update: Here is a datasheet for an SLA battery. Depending on whether you use liquid or Gel electrolyte will dictate whether they can be used in any orientation. Most are Gel based and can be used in any orientation.
The batteries do have a vent, but it can only be popped due to abuse (typically high temperature overcharging). They are considered indoor safe.  




Since you'd like to speed control the fans you need a speed controller. I'd suggest that you DO NOT get a PWM speed controller as some BLDC fans may operate poorly using these. Notice here in the Arctic data they simply point out the operation of the fan at several DC voltages. Since you've pointed to a 3-pin fan you might be able to use a computer style PWM controller, but you need to get one that always starts the fan on a high voltage and then reduces it. Otherwise if you turn the unit on and the speed is set low, there is a chance the fan will stall.
The fans you've selected have an amazingly low start voltage (2.6V) so maybe you'd be OK, but I would be nervous.
Arctic have the F8 4 wire PWM fan (available on Amazon) for almost the same price but they are just slightly smaller. Here's a link to search Amazon for suitable speed controllers.


If you have not yet committed to the fans, I'd suggest that you get 4 wire fans. These have a separate speed (PWM) signal, and are internally intelligent enough to ensure they start at whatever speed you set.
The really nice feature is that you can run the fans on 12V (or at least the 13.8 - 10V range you use an SLA battery) all the time, and simply send a digital signal to tell them the speed to run at.
As you can see from the Arctic data on the F8, you don't need to change the supply voltage (which is what the high powered 3 wire PWMs do) and can still reduce the speed and the power consumption. 

 

Arctic also show an F9 PWM fan, and it's not that much more expensive, but I don't see it for sale other than singly on Amazon.