A word for the wise. I haven't checked to see if this glitch was fixed, but be wary of stacked upgrade buffs. I once had two emerald buffs going at once. For some reason, the buff stack only was in effect as I took one of the upgraded items off, not when I put it back on. So with that vitality boost decreasing each time, I, with curiosity piqued, got my health to zero due to having zero vitality. Just saying, be careful with taking off an item with a buff that you have stacked, for I don't know if Krin has fixed it since I sent him that bug report.

As for the limits of upgrades, if you get the premium content, you can buy as many as you want, for a decent price as well. There are also...two(?) new gems, ruby and sapphire I believe. I remember ruby for sure. Aside from premium content, you're limited to the non respawning ores and the ones they give you throughout the game. I actually missed out on a shadow gem, since when Shigeru tried to give it to me, my inventory was full, but it didn't say "your inventory is full, come back later," it just "gave it to me" and it was nowhere to be found. Guess that's what I got for looting :P

Just realized you can mine a crystal in the bamboo stalks passageway where you get deep water too!

Hum, you can get two Shadow Gems as treasure, one is a gift from the blacksmith, and another one can be found in a chest in the third (I think) floor of the Emerald Temple building.

Also, Lady Shiori has a good chance of dropping Emerals, and Kazuro has a quite low chance of dropping Crystals. So I assume that there is a way to drop Shadow Gems too, but I haven't found out yet. To me, the most likely hypotesis is that Kazuro can drop both and I have been unlucky so far, because it seems weird to drop from a weaker boss, while Shadow Gems are, in my opinion, the best component.

because it seems weird to drop from a weaker boss

Whoa whoa whoa, you think Shiori is the weaker boss? I stomped Kazuro into the ground, but, if nothing else to make it harder, had to be overly cautious with Shiori.

Lady Shiori's difficulty was mainly to figure out a consistent way of dodging her bolts, then she was quite repetitive in the fight. Kazuro, though, had no easy strategy, and he could always freeze me, walk to me, stun me with his shockwave, and kill me in two hits, so the fights were quite intense and I had to time my Meditations very well to dispel the freeze effect.
Anyway that's not exactly what I meant :P I've killed lady Shiori a lot of times and I'm fairly certain she doesn't drop anything but Emeralds and Naga armor, but I can't say the same about Kazuro. So to find the Shadow Gems I would have to try the bosses before lady Shiori, but it sounds more realistic for them to drop from Kazuro rather than a boss weaker than her.

Whoa whoa whoa, you think Shiori is the weaker boss? I stomped Kazuro into the ground, but, if nothing else to make it harder, had to be overly cautious with Shiori.

If you don't look at her when she uses Serpent Gaze she is quite easy to tackle, other than dodging her poison bolts like gaboloth said. However, Kazuru always is fairly easy for me as I always save up all of my food and  potions (those delicious elixirs!) for him.

Could someone please tell me where the upgrade guy is or i cant find him :/

nevermind found him sorry guys

I actually can't find the weapon upgrader guy, i can't find him!!!

In which town is the upgrade weapon guy in?