What can I subistute for LYE in soap recipies?

Answer 1

You can't use anything else in it's place.
You should be able to buy it at your local hardware store (many groceries also carry it). The most popular brand is Red Devil Slaked Lye.

Or you can make your own by running rainwater through wood ashes and then catching and cooking down the water. (I'd buy it instead)

Answer 2

You won't have soap without the lye-- it isn't harsh actually, and your usual receipe will have softeners and such.