Welcome to the forums! Hate to say it but that might be a tall order. I sell Dennerle and I've never seen their glass tops offered as a stand alone item. If you have a LFS near you, I'd ask them to check with their wholesaler and see if they can source one. Those tanks are packed really well for shipping, but I bet some break here and there. When that happens, the filter and light are typically scavenged out for resale; surely there are tops out there from cracked tanks. But as far as I am aware, there's no way to get just the top even as a special order. Shipping a pane of glass is less than ideal so I'd try to source one locally if you can.
Nothing good happens fast in an ecosystem.