Mehron says they no longer sell metallic powder in large quantities i.e. Kg, only in one ounce bottles. Is there another supplier for amounts that allow messy play, thick mixtures, dunking etc?

I honestly didn't know they sold them in anything but the little bottles. I see it sold in bulk containers by wholesalers like Northwest Graphic Supply that seem to be at least art-adjacent and not industrial, but it's not like the kilo bricks of gold powder I've seen shown off by Japanese producers.

I guess there's no real demand in that application.

You can buy equivalent metallic mica powders (ie ground metal powders) in 1kg from specialist craft and paint suppliers but it's not really any cheaper per volume and little consideration in its composition would have been given to human contact (although it's essentially the same - like buying non food grade peanuts).

Since this type of pigment remains opaque mixed with 95 percent medium (necessarily an oil or solvent) the thickness and texture of the substance you create is mostly reliant on the medium. Adding too much more than 5 percent seems decadent but is logically pointless and astronomically expensive.

I don't know what the solid bars constitute but they are logically mostly a vehicle or medium (a solid like butter or wax?)

Mehron I first tried long ago, but it's expensive. So I did research and found Kamam Pigments , an art supplier here in Canada and used their products a few times. Better value, it's the same stuff as mehron or Graftobian. Messygirl had mentioned years ago to a question in forums that they get it from here, NNI Group. I had saved it long ago.
https://easyleafproducts.nnigroup.com/product-category/powders/page/2/

Recently I had ordered metallic powder in the winter from AliExpress and recently tried it. If you search, Bronze Pigment Powder, you find some. There are finer grades available from what I got. Worked well and I was happy with it. I had .5kg I tried using mineral oil and it had a nice texture and slip. The mineral oil I got from Walmart, it was very inexpensive and clean, no odors. It is sold for constipation lol. I didn't want a seed oil because they oxidize and have smells. I had used coconut oil before because it doesn't irritate eyes like olive oil. I'll stick with mineral oil now.

I did do some reading on how to thicken oil. I had been curious before if it's possible. I found out, stearic acid can be used. It is used in cosmetics and personal care products for this purpose. It is a long chain saturated fatty acid from food type sources like fats anumal fats or plant sources. There are videos on how to use this on YouTube on DIY channels for making lotion and such.
A lady gave some example of mixes at 5, 10, 15 and 20% By how it looked 5-10% would be good for our purposes and shouldn't cause any irritation or problems. It is an expensive material, so I'm just going to try 5%, and that looks like it would thicken quite well with the powder. You melt some in some oil first, then pour it in mixing into your the rest of oil. Powder after.
I had seen also, amorphous silica can be mixed into oil as a thickener . This is used in paints as a filler, to change viscosity, and opacity for the paint to cover. This is mixed into the oil phase first before being added to the emulsion. It must be mixed with a high speed to break the small particles up. We could use a kitchen hand blender or blender for our purposes. I got a ninja, and a cheap hand blender that may not be sufficient. Let's cook! Lol You can see demonstrations of the material on YT, mostly industrial. Now with this one, I don't think it will be nice for the eyes being it's a tiny dry silica chunk, crystal like in geometry. Probably Sharp and irritation.
So it makes sense as an option to use the stearic acid. A little is expensive, but not too much is needed. It is a natural fat. You can find this on diy soap and lotion stores . Amazon is not necessarily a better price.
Attached is a picture from the store I got the powder. You can find finer grades. The one I had was nice. The geometry is larger almost flakes with a mirror sides. Mehron I tried long ago is a finer smoother powder, so I'm curious to try one listed as fine, or I saw 1000 mesh out there. I wouldn't want to go smaller because then you get a lot of nano size in there and I don't want that migrating around.
Savor your golden moments.

https://youtu.be/7QlC_c35t54?si=eqSJl0RwOehJoqCd