Black treacle is a very British product and it would be rare for a UK supermarket, large or small, not to stock it. Elsewhere in the world, it can be harder (and more expensive) to find. I'm told that molasses makes an acceptable substitute for this treacle toffee recipe. Though keep in mind that black treacle has a distinctive bitter taste that regular molasses will lack. This will, of course, affect the final taste and colour of your hard toffee. Blackstrap molasses has a robust flavour with an edge of bitterness about it. It is perhaps a better closeness to the flavour profile of black treacle, so it's worth giving this a go if you have some to hand.