Bread – Turning toasted, non-crumbly whole wheat bread into breadcrumbs

Industrial breads use 'dough conditioners' that soften the dough and make working with it easier. You can buy such mixes online, here for instance, and I've seen them for sale in natural food stores and the like. Other things that might work are adding a starch or a fat, or heat treating some of your flour in the microwave (a minute for a cup, don't do this to all the flour, it destroys some of the gluten). Guar gum or xanthan gum will help to keep things moist as well.

I've never used these techniques, so some experimentation might be necessary. Store bread in plastic bags as soon as it cools, and don't slice right away.

Dry biscuits (American 'biscuits' - not cookies), or anything other flour based quick bread that dried out - if you're trying to replicate the bread crumbs. You may be able to get by with a modified muffin recipe as well. You'll want to avoid fats and oils to reduce the moistness.

Yeast bread develops a particular flavor and larger hole structure than quick bread though. The flavor of a quick bread breadcrumbs should be slightly different, but in many dishes the subtle difference won't be overwhelming unless the breading is a 'star' of the dish. The hole structure really doesn't matter luckily - as developing that is a large part of the time factor of real bread.

Drying it out will be the harder part. The denser structure holds moisture better, so slice it thin and give it low, dry, long heat.

_{Personally, anything that you're going to make for the purpose of breadcrumbs that isn't just a substitute (like cereal, crackers, etc in the other answer), sounds like it'd be easier just to make a loaf of fast rise bread to me.}