Sure, there are 2 relatively reliable shops in Islamabad....I don't really trust the ones in Pindi....there's one in F-6 Super Market called Famous Toys....their ph # is 051-2873453....the other one is Kiran Electronics....their ph # is 051-2650175
About the Xbox 360....yes, you can get it flashed....I'm not sure how much it costs these days, but it shouldn't be over a 1000 to 1500 Rupees
Feel free to ask if you have any further queries