Prefixes can be bought.
There are several ways of doing this, either visit a city merchant or one that sells items and names for one of the four factions you do city writs for. (could be wrong here, but I do know that the city merchant in Qeynos Harbour sells the prefix titles, and so does the vendor in the Mage Tower in South Qeynos)
Be advised, it is all linked to the guild level (with the four faction parties, you will also have to have a certain faction, not sure if it has to be ally) and they are expensive, 1pp 50gp and a load of status points too for the "Sir" prefix for instance.