Making your own modelling slate chips for basing and terrain couldn't be easier and is a lot cheaper than buying them from a hobby store:
You should be able to find a natural roofing slate quite cheaply from a D.I.Y. store. Or as in my case, I found some dumped down a back alley.
Just wrap your tile in some cardboard, I had to break my tile in half first. And then take your smashing tool of choice to it, for me it is this weird cheap-ass hatchet/hammer/crowbar tool.
Check on the tile and break it up a bit by hand along the fractures, start laying it on top of other pieces to add more stress areas. close up the cardboard and smash away again.
You should get a large variety of chips You can dump the lot of this into a large tub and leave it at that or like me you can go an additional step.
I sat at a table and using a sieve over some paper I was able to sort the slate chips into seperate tubs of different size gradients. Large chips, Medium chips, Small chips and collected a tiny gravel too.
(I recommend you wear a dust mask during this step)
I've added a 28mm Chaos Cultist here for scale, as you can see I've got plenty of modelling slate chips to last me quite some time. Total cost for me on this was a couple of hours effort.
I hope this quick tip was useful.