Going along a different, more story based, harvesting path (will's answer covers harvesting by yourself pretty well); from the link you shared...
A adamantine golem’s body is made of more than 4,000 pounds of adamantine, mithral, gold, platinum, and other metals worth a total of 100,000 gp.
and under the requirements
Requirements Craft Construct, crushing fist, geas/quest, heal, stoneskin, wish, creator must be caster level 20th; Skill Craft (sculpture) DC 35; Cost 350,000 gp
And while the description is just ignorable fluff, it does say
The vast amount of adamantine required to build even one of these destructive golems is so significant that most worlds do not have enough resources, forcing the creator to travel to the Plane of Earth or remote Outer Planes simply to gather the raw materials needed to build the golem’s body.
Interestingly, despite the description, the RAW says as long as you have a piece of adamantine, mithral, gold, platinum, and enough other metals (like iron) to hit the 4k lb and 100k gp marks, you've satisfied the requirements. The fact that you included the cost of buying 100k gp in raw material in your construction cost (since harvesting 100k gp in anything by definition inflates your total assets by 100k gp) implies you want to be as lazy about this as possible.
Befriend the Merchant Guild
Whether it's a guild, association, or loose network of reliable people; the merchant's guild will be a valuable ally for getting connections for purchasing large amounts of raw material. You can even approach the local smithy to help you get in contact with their supply chain (since even common metals will contribute). The bigger the guild, the more likely they will know a guy who can help you get your hands on the rarer metals, and you may be able to secure a bulk discount from the mines directly.
Even if you want to be fair to RAI and use mostly adamantine, you can team up with high profile individuals in the guild to do a harvest run with (Transport caravans, heavy duty mining magic/equipment, harvesting areas too dangerous for your party alone, etc.). This will allow you to harvest more in few trips, and everyone profits. (Efficiency through mutual benefits)
Befriend the Adventurers Guild
Similar to the merchant's guild, the adventurer's guild will be useful for connecting with people of higher level magic / fighting capability. They also should have a vast trove of useful adventure information, like how to obtain large amounts of precious metals. Similar to the merchant's guild, you will want to use your connections here to find/tackle risky/high-profit adventures.
Befriend the (Black) Market
With the sear amount of material you need, you will probably want to pick up as much material as you can in every city you pass though. If you can get a connection to the black market as well, they will have more of the rarer metals (though, beware of scammers / local law enforcement; The DM has much more control of story based connections than class feature ones).
Create your own Organization
You are going to need a safe place to keep this hoard while you are in the stockpiling phase. Even with bags of holding, due to limits on the bag, it won't be easy to keep all the material with you. By establishing your own stronghold, you can keep your stockpile safe their, while obtaining a (small) army of (low level peasant) fighters that you can deploy to extend your networking influence and help reduce transport costs.
To be honest this one is kind of a lot of red tape/work. Definitely talk to your DM to make sure they are ok with it, and establish how much attention/effort will go into this side project.
Befriend a Deity/Demon/powerful-entity-from-beyond-the-veil
This will require the DM to sign off on, but powerful godlike creatures could easily harvest and give you the material. They also obviously won't do it for free, and they have no interest in gold. How you pay them would be up to you and the DM, but depending on the DM can turn into a really interesting side story. Especially if you agree to a magically binding blank check (like an idiot)!
(If shenanigans are okay) Kill your character
You know what is more efficient than gathering resources? Multiple people trying to achieve the same thing! Be a loose conglomerate of associates for golems. When the total stockpile is reached, start a winner takes all war and the sole survivor becomes your character.