Is there a substitute for chocolate graham crackers?
If this had been a graham cracker crust, I'd have recommended adding some cocoa powder to the melted butter when making the crust.

In this case, as you're just using the crumbs in a layer of a parfait, you'd want the flavor to be better integrated as it doesn't have a chance to cook.  Therefore, I'd go with either crushed chocolate cookies or a mix of crushed graham crackers and chocolate shavings.  Both will give you the chocolate flavor and crunch at the same time.

You can also try working more chocolate flavor into some other ingredient (either with a chocolate liqueur or cocoa powder), but with something layered like this you often want the contrast of different flavors in each layer.