I'm late to the party, but I thought about using a liquid drain cleaner too when I first started out and had trouble finding solid lye. When I checked into the ingredients of the liquid product, however, I realized there was way more water and not enough lye in the mixture. It would have been really tough for me to make decent soap using this kind of product.
Now that I've been soaping for a few years and participated in the "lye heavy soap" thread a year or so back, I am pretty sure I could make soap with a liquid drain cleaner, but this is a challenge better suited for an intermediate or advanced soap maker, not a novice. And I'm not sure I'd do it anyway, even though I think it's do-able with the skills I now have. Commercial liquid drain cleaners are often not just water and lye. The risk of weird things in my soap is just not worth it.