dont know about islamabad but from chah-e-sultan rwp u can get it done, but take good care in selecting the brake booster, try to buy a piece which isn't being opened. Its a hard task as people at chah-e-sultan are very rungabaz unless u r lucky and find a dependable person.
One good solid brake booster not repaid ll cost u from 2k to 3k depending upon it condition.