Are you talking about the plastic/resin trim that is glued along the top and bottom of the tank?
Or are you talking about a hood/canopy to house lights and stuff?
Back in the day (oh about 10 years ago) the bigger LFS's would have trims and mouldings available premade for popular dimensions, or by the foot and they could cut them to length. They also sold the silicone sealant needed for it.
I have never seen them in the big box pet stores that are in my area though.