The dark one is not comparable to any of the darker invert sugars, so I wouldn't recommend it, if you're not after that particular flavour it provides.You can get Lyle’s on Amazon. Light & Dark.
I will brew tonight or tomorrow and will do a little experiment on the side. I will need about 10% of sugar for this recipe, so I will try to make some cheap homemade invert syrup while mashing. I got raw cane sugar and some raw and completely untreated one as well. I will slowly heat it up with some lemon juice and see where I'm at when the mash is done.