Try looking in your yellow pages for other electronic stores.
I sometimes buy from Electronix Express located in Avenel, New Jersey. Unfortunately they only do mail order but they will have the parts you need. Call them at 1-800-972 2225. Maybe if you live close they will let you pick up your order. Their web site is www.elexp.com. They sell the hex inverter you want for 24 cents, but since they only do mail order you will have to pay for postage.
Have you tried to call other Radio Shack store in your area. Even if the part is discontinued there still may be a few left in other stores.
If you can't find those parts you can make the Clark Zapper instead. Radio Shack will usually have those parts except the LED part # 276-044 which has been discontinued. Try substituting it with part # 276-209