I'd try the radioshack route for the DC-DC @ 3A... if you blow the supply just return it as "Defective" (yeah I know it has a fuse) If you're not going to push the requirements (i.e. barely anything) you should be able to get away with 3A. You can prolly up the fuse just a smidgum as well to provide maybe 3.2A or so if 3A is just barely cutting it.
However I don't know how "regulated" the DC-DC radioshack supply is, especially when you are starting the car or stuff like that. I'd investigate that first before plugging anything in.