The best way to go about it so you don't over or under-spend, is to calculate your desired power load. A sealed AGM battery would be ideal indoors. (NOT a gel-cell, and NOT a car battery. At the very least, use a hybrid marine deep discharge battery)
1) How much current does your scanner draw? For instance, a Uniden 996XT draws 900mah. Let's make that 1 amp / hour for convenience.
2) I want to power it long enough to sustain me through a 12-hour outage. So 1amp * 12 hours = 12 amps drawn.
3) DOUBLE that requirement, so 2 * 12 = a 24 ah battery is needed.
The reason for doubling the requirement is that the battery will cycle much longer if only drawn down to 50%. This may not be a huge issue for one-shot emergency where you will dispose of a battery drawn so far down that it is dead. However, doubling the capacity also ensures that you'll have enough operating voltage to properly operate the scanner. Some pieces of gear don't like going below 12v.
Depending on how much power you draw, even those convenient all-in-one battery packs may be too small, and even D-cell packs can be too small.
Assuming you get ac restored, you can charge it back up with a quality charger like a NOCO Genius 3500/7200, or say a Schumacher SC-1000AP charger that can accomodate both flooded and agm batteries. But how much of a charger do I need?
Flooded batteries don't like much more than 10% of the total capacity for recharge - ie, a 70ah battery won't like much more than about 7 amps of recharge current. Sealed AGM's can take up to about .25C, so a 70ah agm battery can withstand an initial 17.5 amp charge. You can see that the agm will charge up faster if that matters. AGM's also have much less self-discharge, and only need topping up every 6 months or so if you don't trickle them.
Buying batteries - try not to buy them if they are more than 6 months old as listed on the battery. AND / or, do not buy them if you receive them and they are under 12.6v on a multimeter if you can avoid it. If you are willing to chance it, you can go maybe a year, and perhaps 12.4 volts, but that's cutting it close when purchasing that you are getting old stock, or something that has been sitting in a hot rail-car for months on end.
Radio Shack has sealed AGM batteries in their "UPG" line, although these ups-style general purpose agm batteries are easy to find elsewhere.
Tip: if you do get a dual-purpose marine battery that doesn't directly list the AH rate, but only the "reserve capacity", you can multiply the "RC" value by 0.6 to get in the ballpark for calculating your needed capacity.
You may want to look at this helpful battery faq:
Deep Cycle Battery FAQ
Solar is also an option, but do homework first since many vastly understimate the amount of panel capacity needed to charge a battery in limited solar insolation timeframes - aka "deficit charging" - something to be avoided.
