I've never seen it in that size range. I have seen pumice in an orchids size. I think pumice is preferred for plants,it is softer than lava rock.

I've seen some that size @ HD - But it's in little jars - like 16oz size, for around $5USD. Wish they sold bigger qty.

That is what I use. I get 20Lb bags at a local supplier named bushel stop. I don't think they ship...Check with places that sell mulch.

Black scoria might be a little difficult to find in that size for a reasonable price. The usual stuff one sees, at nurseries which have bulk bins for mulch/rock/dirt/sand etc, usually is the 3/8" size.

The red lava is available at the size you want if you can find a seller.

"The size of a dime" is pretty small for red lava rock. I have bought some small red lava rock and medium size rock online. Just search for it, maybe Amazon, and sellers will pop up. Pretty expensive in small sizes though. I have also bought sizable bags at Home Depot, sorted and sifted through it for small-to-medium size pieces and tossed the rest.

Online will be expensive because of freight. Garden centers bring in truck loads. Local will be the best price if anyone has it...

What´s a dime?

It really isn't this big. https://www.google.com/imgres?imgurl=https://upload.wikimedia.org/wikipedia/commons/3/3c/Dime_Obverse_13.png&imgrefurl=https://en.wikipedia.org/wiki/Dime_(United_States_coin)&h=2000&w=2000&tbnid=ftsG7_U9bqfirM:&q=us+dime&tbnh=186&tbnw=186&usg=AI4_-kQBquoBL3JvVy0EaS-kun9KqmnvQA&vet=12ahUKEwi5sq-j3vLfAhUCHzQIHcxDBZ0Q_B0wE3oECAEQBg..i&docid=qZg1uCNPvf4DkM&itg=1&sa=X&ved=2ahUKEwi5sq-j3vLfAhUCHzQIHcxDBZ0Q_B0wE3oECAEQBg

I use Stalite which is small, grey stone. I buy it at orchid shows.

Mike, you should hit any orchid shows in your area. The vendors sell lava rock and its much cheaper than online.

Jane

Here are several options, not sure if the price is good, but there are options!

https://www.repotme.com/orchid-potting-media/Volcanic-Rock-Small.html

https://www.repotme.com/orchid-potting-media/Volcanic-Rock-Medium.html

https://www.repotme.com/orchid-potting-media/Volcanic-Rock-Large.html

I would suspect it might be easier to get Hydroton or similar puffed clay aggregate balls (typically used in hydroponics) in that size range.

Lava rock is mostly mined in western part of US and it is much cheaper there. In eastern parts it is much more expensive no matter where you look. They have to ship all the way from there even if it is in bulk. If your need is about a bag full (say a grocery bag size) then your best bet is to buy a few bags of lava rock from HD. They are biggish. Then smash them with a hammer while they are still in the bag and you can get what you need.

Alternatively, you can go to your local landscape supplier of lava rocks and beg them to sieve a bag full. I have done that a couple of times long time back. Usually they are quite cooperative. I take a home made sieve made of hardware cloth and request them that I will sieve it myself and they usually find it quite amusing.

These days I just cough up the price. I pay about $35 for 1/4 inch sized lava rock for a 50 pound bag from a local bonsai supplier. About the same for pumice too.

Mine is red lava mined from Colorado. The bag is about 25 or 30 lbs. and it costs around $8 or 9 dollars. The supplier brings in the bags on a semi. They have small medium and large. The small is too small for me. I get the medium...

I was looking this up a while ago. Thought I would include lava rock in my soil but I went with other things instead.
So I don't know about dime size but if you're looking for a quarter inch size, here:
https://www.amazon.com/Bonsai-Jack-Maroon-Gallons-Quarts/dp/B00GPLXBBI/ref=sr_1_5?keywords=lava+rock+bonsai&qid=1548182449&sr=8-5

Bonsai suppliers and hydroponic shops.

Of all places, I found some at my local greenhouse! Thank you everyone. I really appreciate it.

Mike

How does one use lava rock?

i live in NM and see it while walking my dogs.