If it is an anode, I seriously doubt you will find it anywhere other than a RV dealer, you should be able to find it online, or on ebay and they will ship it to you, as long as the tank has a plug in it, you can use it without the anode, but beings they put it in there, I would pick up another one and install it. I have to rinse my HW tank out about once a year due to mineral in the water, which is really no big deal, I do it in the fall when I am putting the trailer to bed for the winter.
Wim, when you own an RV, you learn that the internet is your friend, there are some many things that I can't find locally and I just pick them up on ebay.