Sorry for the delay, I was traveling.

I found a cheap, low watt version from Amazon. https://www.amazon.com/gp/product/B005440HLO

Keep it turned way down or the water gets too warm. Nice thing about this one is suctions cups keep it in place. Mine was about half the price at the time, says something either about the economy, the dollar, or Amazon. You choose.

You could probably just go into any pet store and get something off the shelf that would work nicely.