there are a few ways to do this, I`ll cover as many as I can without trying to bog you down with stuff.
1) Do dungeons with monsters at a level equal to the deconstruction skill you have /5 ie if deconstruction is at 25 you need level 5 or below as they will have the items you can dismantle and lower.
2) Raise a few skills at a time while working on deconstruction, let me cover the choices
2a) Geology, to collect gems. Then raise alchemy by using the empty potion bottle plan and stonemasonry with the powder plan. Both these use 1 gem and dismantle back into 1 gem no loss of items. Also raising alchemy allows you to use Transmogrify which will lower the level of materials if you need it done
2b) Mining, to collect ore. Then raise blacksmithing by making ingots and from those ingots either rivets during the skill gap or Tinkering and bullet molds/arrowheads/dart tips or bolts as they all use 1 ingot and again no loss of items
2c) Lumberjacking, to collect logs then carpentry and small stock. You can also use the plank and small axe handle, again both these use 1 item and therefore no loss of material
2d) Skinning, to collect hides. Then tailoring and plain handle, again using only one item. All these use only one item so you don`t lose anything when you dismantle
3) Purchase items from players to allow you to dismantle them into base materails.
There is no difference in skill gain between slow and fast dismantle, you get the same amount of materials no matter what if you use more than one item to build you will get a random amount back upto the amount you used there is no way to determine how many you get. Slow dismantle will give you xp, that is it`s only advantage.