How can I make breadcrumbs without a full, yeast-leavened loaf of bread?
Three substitutes come to mind:


Saltine Crackers 
Pretzels 
Select brands of Cereal (Chex, Corn Flakes, etc.)


Take these for a spin in your food processor till you reach the consistency you desire.

Depending on your ultimate goal one or any of these could serve as a suitable sub for "bread crumbs"