What's a good spot for farming money and another spot for fulminated mercury?

[DuelShockX]

I know only the gun clan drops the mercury after you progress the game but does anyone have a recommended spot to farm them? Also a good spot for farming money?

[destro]

Ashina castle idol, use golden vortex prosthetic and make sure to activate the demon bell and use mibu possession balloon and mibu of wealth for better drops and money.

[Veritas]

It's just the gun fort clans and the interior ministry red samurai that will drop fulminated mercury when you have the demon bell

[Alexoferith]

In the early stage, after defeating Genichiro at top of Ashina Castle, you will have to go to the Poison Pool. Hopefully you have already defeated the Snake Eyes there before defeating Genichiro. If you haven't done that, then this will be difficult. It is because putting the Snake Eyes aside, all the other minions will be gone and replaced by 3 dudes that are holding a canon. You will see what I mean if you go to the Poison Pool Idol. Grapple up and you should see one guy with the canon with his back facing you so you can stealth kill him easily. Then have a good look around. You will see another guy with a canon patrolling the central "island" in a circle. Then at some distance away slightly towards your right, you should see the 3rd guy with a canon. If you haven't got rid of the Snake Eyes before defeating Genichiro, the whole area becomes quite difficult. Yes, you can use a green sugar to go invisible and grapple across to kill the one on the far side and then jump down to kill the last canon guy. But by now, Snake Eyes will spot you and her shooting is quite accurate.
Ok, let say Snake Eyes is not there (assuming that you have killed her before defeating Genichiro), after arriving at the Poison Pool Idol, you grapple up. Now use the green balloon which increases the drop rate of rare items. But you will have to have the Daemon Bell as well. This will further increase the drop rate of rare items. However, the drop rate will only be around 1 in 30 or even as high as 1 in 50 to 60 (I don't have the exact figure but from my personal experience, if you don't have the Kuro's Charm, the drop rate will definite be around 1 in 30). These canon dudes do drop the fulminated mercury but with the drop rate in mind, you really have to have quite a few of those green balloon, at least 10. However, this will only allow to farm just enough to upgrade ONE prosthetic tool, i.e. about 3 to 4 mercury.
You can pop a green sugar to go invisible if you are worried about being spotted. But from my experience, if you time it right, you won't get spotted.
Saying all these, there are two ways to farm. One is to go around and kills ALL 3 canon guys. Always do it in this order, the one nearest to the Poison Pool Idol first, then the one at the far end of the area, and the one at the centre area the last. Then hop back to the idol to rest. Repeat. You should be able to do at least 3 round before the effect of the green balloon runs out. Oh, never use the personal idol as this will cancel out any effect left of the green balloon. Always hop back to the Poison Pool Idol and rest there.
The 2nd way is to just killing the canon guy nearest to the Poison Pool Idol. Then hop back to the idol to rest. You should be able to kill him 5 to 6 times before the effect of the green balloon runs out.
Both Destro and Snazzy Dragon are correct. The area they mentioned will drop mercury but this is the final stage of the game. But it will be much harder as you will have the Daemon Bell plus without the Kuro's Charm especially fighting the Red coat samurai with twin blades. Destro's method is the safest.
Hope this help. Good luck and have fun. :)