Shaun, this seems to be as rare as hens teeth Sir. I would contact Glock Customer Care for a G40 Slide at 770-432-1202

Your best bet would be to check out $un&roker- just replace the $ with a G and the & with a B, sorry loophole to not have my answer deleted. They let private owners and small shops sell on their site so they usually have rare and hard to find firearms and accessories. My friend was looking for a Glock 17 and 19 slide and that's where he found his. Be prepared to pay a decent amount for it though at any place, for some reason a complete slide with barrels costing close to what a new complete Glock pistol costs. Think it's because you can buy 80% lowers to build them so they make sure it costs an arm and a leg. Hope this helps.