Where do I get what I need to fix the plumbing

Hint 1 of 6:

Did you read the how-to book in S.U Student Union's closet ?

Hint 2 of 6:

Did you take the coupon (from the book) to the plumbing store ?

Hint 3 of 6:

The store is just south of Barmaid U. GIVE COUPON TO PROPRIETOR.

Hint 4 of 6:

Did you attend the first day of Alchemy 302 ?

Hint 5 of 6:

Then you should have turned the iron lump into copper.

Hint 6 of 6:

That's all you need.  FIX PLUMBING.