I buy that pink foam insulation stuff they sell at HomeDepot. It's either 8' long & you can cut to size. Personally, I have 2 layers under my 100g acrylic. Very inexpensive but well worth it.
Sorry for your flood. Actually, I'm glad you posted this thread because I was offered a great price on a "15 years old tank" but now I know I made the right decision to turn it down.
Anthony