Everything (and i do mean EVERYTHING) that uses rechargeable batteries these days runs on 3.7v, from Nokias from 10 years ago, up to the latest tablet computer - pull one apart and you will find two, or three, lith-ion 3.7v batteries daisy-chained together. My bike lights run on 3.7v cells, which are £1 each, or, I can dismantle an old laptop battery and get six of them.
I had a PDA that ran on an old Nokia battery for over a year after the original died, it recharged just fine from the PDA's original charger. (Eventually I dropped it, the sceen wasn't worth replacing.)
Most devices state a 5v input - this is to allow the device to charge more quickly, and to let you use it WHILE it's charging. The mAh number is just an indication of the lifespan of the output - the higher the number the longer the battery lasts before it goes flat.
The PROBLEM you will run into is low-quality construction of aftermarket batteries. Cheap Chinese (this is a little stereotypical, but also has too broad a stroke of truth to ignore), imports often overheat or explode, because of leakage across the layers of the polymer sandwich that make up the battery. Any battery that costs less than the shipping fee is probably an aftermarket 'fake', which is either counterfeit, or was rejected from the real manufacturers production line.
If you REALLY want to keep your kindle alive until you can get ahold of a decent quality replacement, grab an old laptop, make sure the battery is flat, strip the battery apart and pull the cells out. I've done this with a few, flat is safer!
http://www.ebay.co.uk/itm/Li-ion-Bat...item33846b4a1a will charge those puppies quite safely. I use one myself all the time.
http://www.ebay.co.uk/itm/Double-USB...item56587bc774 will then take the charged up cells, and supply 5v DC to charge your kindle up! - This thing will also charge by USB, not sure how quickly it will do it compared to a charger driven by the mains.
I use these things for EVERYTHING, bike lights, phone/kindle/camera travel charger etc.