I have a new Rheem Model XG40T09HE40Ho gas water heater.

try looking on the manufacture's website or contact the seller

Sometimes the instruction manuals mention timers. Just out of curiosity is it a rental or did you purchase it? The reason I ask is because a tankless model only heats on demand, and if you are renting, you could switch it out. I just got one and love it, no timer needed.

p.s. This company says they sell different types:

http://www.gaswaterheatertimer.com/purchase

I would check with Rheem first like Elizabeth said, for possible safety and warranty issues. http://www.rheem.com/contact.aspx

Having been in the water treatment business for 20 years I learned a lot. I had a Rheem gas heater, and bought a 2nd one. Being FRUGAL, I bought a conventional, likely at any home center , timer. I set it to have hot water for OUR use periods and OFF and this is really important BEFORE, you use the hot water. The idea is simple, you want a water heater to store COOLER water, NOT hot. The heat loss is far less that way. So in our case, my wife uses it about 8 am for an hour or so. That timer is set to come ON at 7:15 and OFF at 7:45. Now she has a full tank of hot water. When she is through, it now stores "tepid" or Less hot water. Then the earliest we use it in the evening is about 4:30 or so. It is set to come ON at 3:45, then OFF at 4:15. Evening showers for me and a bath for her, plus dinner clean up and even the dish washer, now leave totally tepid water in the tank until the next morning. Think about it. STORE cooler water, less losses. I have been doing this for 30 or more years. On laundry day, IF the projected loads require, I simply turn it on manually for 30 minutes, BEFORE the laundry is launched.