Have you read the thread that Fish linked to in post #2 on this thread?
The important characteristic you need is high current. A few volts will be enough, unless your wire is very long and/or has a very high resistance.
BTW if you can get different grades of nichrome wire, go for one with relatively high resistance (high ohms per foot). They will need less current to achieve a particular temperature. (But they will need more voltage. So they are more suited to your 12V/2.5A supply.) (Although I agree with Fish that a charger is probably not the most suitable power supply.)
You could use a linear regulator but 2.5A may not be enough for your wire, and the regulator will need to dissipate a lot of power. For example if your wire needs 2.5A to get hot enough, and at 2.5A the voltage across the wire is only, say, 2V, the regulator will be dropping 10V with 2.5A current flowing through it, which means it will dissipate 25 watts (multiply voltage and current together.) At 25W you will need a biiiig heatsink on that regulator. A switching regulator is much more efficient.
A tidy option would be a buck (aka step-down) switching power supply. These are available as assembled PCBs for prices around $10 on eBay. You can power it from your 12V/2.5A charger.
Go for one with an ADJUSTABLE output voltage that can be set down to less than 2V (I've seen one that goes down to 1.2V) and has an output current rating of AT LEAST 5 amps. A lot of them are only rated for 3A and I think that's probably too low for your application.
Here are some possibilities from eBay. Generally I don't recommend buying cheap products from China or Hong Kong and "your mileage may vary".
Here's one that claims 12A maximum output current:
http://www.ebay.com/itm/12A-100W-Ad...882?pt=LH_DefaultDomain_0&hash=item232e38aa32
This one has a constant current output (this would simplify adjustment if you vary the length of your wire) and can supply up to 5A:
http://www.ebay.com/itm/DC-DC-4-5-3...663?pt=LH_DefaultDomain_0&hash=item51b17531c7
This one actually looks pretty darn good. It doesn't have constant current output, but claims up to 15A current and output voltage adjustable down to 1.2V.
http://www.ebay.com/itm/Adjustable-...477?pt=LH_DefaultDomain_0&hash=item19e10b1d9d
No doubt there are others. Those links should give you some idea what to look for.