There are many brands of power converters to provide 12V from 110V out there. IF you are reasonably adept at working on electrical circuits, or know a friend that can trace out circuit problems should something happen, I suggest that you check out as many converters as possible to find one that you can get a schematic diagram for so that repairs can be done inexpensively. Most all parts for these can be purchased at many electronic parts stores, and the most expensive part would be the transformer. This item can be found for as little as $20 to $30. Nothing else in the parts would exceed a couple dollars or so for each. Some would even cost less. Of the last three I had to repair I had to replace a diode (70 cents), a transistor (2.45), and a resistor, capacitor, and a transistor in the third (20 cents, 1.90, 3.15). Only if the transformer is bad, or if a full set of transistors must be replaced would the cost be higher than the above examples. My labor cost on these was only $30 each for two. The third was a freebie.
Voltage converters are really simple devices. They are not like voltage inverters that change 12VDC into 110VAC. Basically all such converters do is to use a voltage dropping transformer and a full wave bridge rectifier to make the 12VDC output.
Of course, there are other options installed into most commercial manufactured converters, but with very little knowledge and ability most any person can make a reasonably dependable converter. The 'fancy' of it would just all depend upon the person finding an enclosure that they can make look more durable and appealing.
It is not the brand that makes any difference. Just stick with name brand parts such as ECG, RCA, GE, Motorola, and such. Most Radio Shack parts are cheap (not much less than branded) contracted parts from China. They are not close tolerance parts.
You can get this at VTS and the new ones like this do not need an additional purchase of a battery minder. Has one built in so it keeps your battery charged properly. You do need to use a battery with converters. You can't just run off the converter by itself.