The needlepoint plastic you can get at any walmart or crafting store the report binders I got at office depot, but you might could find them at walmart as well. And to seal them I just used aquarium sealant. the way I do it is I mark on the outside of the tank where I want to put the dividers and then on the inside I use tape to line off a half inch section with the mark on the outside of the tank being in the middle. This way when you apply the sealer and it dries you pull the tape off and you have a nice clean straight edge. Just make sure that you have the binders lined up from one side to the other, if they are not straight across from one another the plastic can pop out of them from the angle that it is pulled at.
The big problem is my tank is full of water cycling .Does the aquarium seal work under water and the tapes would they work under water?
thank you