Before purchasing a water heater anode rod there are three things you need to be sure of:
(1) What is the clearance you have to install the anode rod. If you have much less than 40", you will need to get a flexible anode rod.
(2) What type of metal do you want to use. The choices are Aluminum, Magnesium and a combination of Aluminum/Zinc/Tin. See the article 'Anode Rod Metal Types' for information on the differences.
(3) What type installation will it be. Do you have a 'hex head' anode rod or is it a 'hot water outlet' anode rod. See the article 'Anode Rod Installation Types' for information on each type.
Did you have any clearance issues? Yes, then you are going to get a flexible anode for a replacement. They cost a couple of dollars more. You will have to make the aluminum vs magnesium decision. An aluminum anode will run around $25, for magnesium it will be closer to $40.
Standard Aluminum Anode Rods
A straight aluminum anode rod is the least expensive choice. They do not work as well as magnesium and may be prone to causing odors in some situations.
Aluminum anode rods are a common type used in new water heaters. There are both health and performance concerns associated with aluminum anode rods.
You can purchase an aluminum anode rod online, at plumbing and home supply stores for around $20 to $25.
Are you having problems with odor. Then you will want to get an anode that is made of a combination of zinc and aluminum. Do not get magnesium. This style will run around $50 or $60. Some manufacturers have special proprietary combination anode rods. It is best to check before you get your heater torn apart.
Hot Water Heater Outlet Anode Rods
Hot water outlet anode rods will cost nearly double the price of the same metal in a straight version. This type of anode rod is made to allow the hot water to pass through it.