What's the cheapest way of obtaining enriched dimeritium ingots and plates?

So I've got my grandmaster diagrams and I'm ready to craft grandmaster weapons and armour.

The thing is, enriched dimeritium plates and ingots is absurdly expensive to buy from the grandmaster blacksmith.

Is there anywhere I can find it (or the parts to craft it) for free? Is there another merchant that'll sell it cheaper?

Thank you in advance and please answer as spoiler free as possible.

The only direct answer to your question about how to get this stuff for free is "random loot", but I know that answer is extremely unsatisfying. So let me enlighten you about a technique I've developed when dealing with the crafters of arms, armors & advanced runestones.

Wealth Recovery

1. Every time you deal with a group of bandits, deserters, etc. you come away with an impressive wealth of arms & armor.
2. Very few vendors are capable of liquidating significant quantities of this wealth outright, but never fear, this is where your stash comes into play. Save this crap for later.
3. When you need something expensive crafted, In addition to bringing along cash and materials, retrieve a pile of junk arms or armor from you stash.
4. Purchase crafted items or additional materials. Feel free to have the materials you need crafted from lower quality materials. FWIW: Deconstructing Dimeritium Shackles will cost less than purchasing the glowing ore outright.
5. Sell junk to recover your wealth and to absorb much of the vendors starting wealth.
6. Repeat as required.

Never leave a smith with more cash than he started with.

Blood & Wine addendum

Since you are looking to craft Grand Master gear, you are apparently playing the Blood & Wine expansion. I've noted that once you clear out the camp of the bandit chief in the center of the northern border of the map, it is repopulated with troops & a very wealthy armorer. Feel free to dump vast quantities of junk on him to claim his cash.

You can craft enriched dimeritium plates and ingots from enriched dimeritium ore. This in turn can be crafted from normal dimeritium ore + orichalcum ore + acid extract. All those components are fairly easy to find.

As it turns out, the cheapest way is to craft those ingots and plates starting by crafting the Enriched Dimeritium Ore. As far as I could tell, there's no way to loot Enriched Dimeritium Ore, Ingots or Plates in the world.

What I did was to grind the money and materials (for the Enriched Dimeritium Ore) by completing some of the points of interest in the Knight for Hire quest. With the money and materials I got I crafted roughly half of the ingots and plates I needed and bought the other half.

I was super excited to finally get my grandmaster gear set until 5h later I found better gear for free...

PS.: Please refer to tjd's answer if you need more help grinding gold.