There are numerous ways to get them. The first two ways are only attainable by finishing the Smoking Kills quest. Afterwards, you need to obtain 35 slayer points in order to get 250 bolts. You can also save up for 300 points in order to learn how to fletch them. You must have 55 fletching to fletch these after your character learns how. The third and most efficient way is to buy them off other players or the Grand Exchange. Keep in mind that you need 55 slayer in order to use broad-tipped bolts.