Most of good weapons can be found only in Brimloch Roon area. Most of them are "specials". Only the Nightblade, mentioned by Bungleau, is always on the bottom of the pool. Here are some sources of good weapons:

1. The Undead Blacksmith in the Shurugeon Castle. He can drop Wizard's Blade, Robinsword, Nightblade and similar good items.
2. The genie of the lamp. You can get three Swords of Argus (the second best weapon after the Wizard's Blade - it's a sword) from it.
3. The pool in the Dragonspire dungeon. There is always a Nightblade there.
4. Centaurs in Brimloch Roon. Not only centaurs at the Skull Castle but all centaurs. They often drop Wizard's Blades or Main Gauches+2. Always check what the centaurs have dropped! Always check all "short blades" and all "daggers"!
5. All "pirate chests" in Brimloch Roon. They can contain Main Gauches+2.
6. The "artifact chests". These chest can contain variety of good weapons: Tonomari Katanas, Battle Maces+2, Wakizashis+2 (the second best 2nd weapon - it's a sword), Robinswords, and so on. Most of "artifact chests" are in the Pyramid. The first "artifact chest" you will meet is the chest with the 2nd Table of Cet in Collasium.

Perhaps a hint for the best tips page; if you want to post on this forum be sure to make a back up of your text on a word document before you submit your thread or reply ;)

As for the weapons, I considered it a little of a flaw that you stick mainly to a few weapons. In fact my solo character had only one sword you can't buy in the armory (Sword of Argus) and one (Assassin's) Dagger throughout the entire game.
I hardly ever found anything useful in a chest nor did any monster drop a powerful, useful or at least surprising loot. But maybe I was just unlucky ;)