You don't need dual gauges on your regulator. The high pressure-side gauge is nearly useless. You should be able to get a single or dual gauge regulator at a homebrew shop, amazon, welding supply, or any number of other places.
Disclaimer: I don't do CO2 for aquariums (yet?), I do it for homebrew, and I have a dual gauge regulator, because I thought I needed it.
The high-pressure gauge pretty much only tells you if the tank is empty, nearly empty, or isn't nearly empty. It will not tell you the difference between a full tank, half full tank or quarter-full tank, as those will all read the same.
The pressure does not drop off as the tank empties, it only starts dropping when you are getting very close to empty. This happens because the CO2 is liquified in the tank. The pressure of the tank will be whatever the vapor pressure of CO2 is for the ambient temperature, until you've drained off enough that all the liquid is gone. Only then will it drop, but at that point you've used most of the CO2 up. I keep mine in a keg fridge at 40F, so it sits just under 600PSI, as that's the vapor pressure of CO2 at 40F. In a heated/cooled room it will sit somewhere between 800PSI (~65F) and 900PSI (~75F). It really makes a better thermometer than gas level gauge.
A decent bathroom scale is a better way, as you can weigh the tank and tell how much gas you've drained. A 5lb CO2 bottle should weigh its stamped tare weight when empty, and tare+5lbs when full.