I see from here that the expansion is available to buy from Steam but there doesn't seem to be any alternative methods of purchase available. Does anyone know if the expansion can be bought anywhere else?
If you want to buy the DLC alone you're out of luck, since it is only available through Steam.
Your only chance to get the expansion without DRMs is through the Most Unholy Edition, the boxed set that contains the base game AND the expansion with no DRM, PLUS a Steam gifting key.
The link I've put here in the answer is for the signed edition on the official store, currently sold out, but it is still available through many other retailers as a quick Google search can show you.