I need some tips on finding the shop buyable schematics. I read elsewhere tarant gunsmith is best, but i keep advancing time and his inventory takes forever (like weeks) to change. Where should I go to find plastique and detonator?
You should advance the time by one day, when the shopkeeper and his inventory are "out of screen". If you can see the chest and the shopkeeper in your screen, the inventory will stay the same.
It depends on how do we define "best". The Tarant gunsmith is as good as the Ashbury one, and as a rule of thumb we could say that gunsmiths sell all the gun and explosive related stuff, quality smiths and dwarven smiths sell weapon and armor related schematics, while inventors are good for getting the more high tech and sophisticated schematics. Both have a 3% chance of being found in a gunsmith's stock each day.