A lot of hardware stores and groceries are no longer selling lye (sodium hydroxide) and there is no substitute for it. Ask your local hardware store if they will bring it in for you.
If that doesn't work, look in your local yellow pages under "Chemicals" and see if there is a chemical supply shop in your city. Go into the place in person. They will probably ask you for your ID to prove who you are. They generally will not sell lye to just anybody anonymously. The downside to the chemical supply store is that you might have to buy fifty pounds of it.

This is the company who stocks hardware stores:
http://www.boyercorporation.com

Call them and ask them if they supply a store near you. If they do not, you can buy from them and they will ship it to you. Unfortunately, it is very expensive to ship lye because it is on the hazardous materials list. So get it locally if you can.