Hi,
I'm not aware of any ready mixed product. My understanding is that most people mix the epoxy, and then once mixed add the filler.
Some years ago I used an Araldite product, I can't remember the product code, but it was sold as a casting epoxy. It had some sort of inert filler, guessing about 60% by
volume, you added the hardener and mixed, which was not to difficult and then poured into the mold. The kit was 1L and it cost about $40NZD. It worked really well,
I was pouring about 500ml at a time....no exotherm and very low gas/void. No mess, no fuss.
Craig