Finding a reliable online source to purchase replacement parts for GE water heaters depends on your personal comfort level and budget. If you want to pay more, you can purchase parts directly from GE, or you can look for discounts on sites such as HomeDepot.com, SearsPartsDirect.com, eBay.com and eReplacementParts.com.
Purchasing parts directly from GE allows you to guarantee the compatibility of the parts with your specific model of water heater, and it gives you a direct warranty on the parts you purchase. However, the parts sold on the site are rarely discounted. Purchasing from GE also gives you a clear method for returning any defective or malfunctioning part.
Alternately, you can look for cheaper new and used GE water heater parts on an auction site such as eBay.com. The biggest difference with shopping on eBay versus buying directly from GE is that eBay does not guarantee any of the items sold on its site. All warranties and guarantees are set by the individual seller, and even then it is up the seller to enforce those policies. You can judge a seller's trustworthiness by reading his seller reviews, which are written by other users who have dealt with that seller in the past. Therefore, these reviews accurately depict the trustworthiness of potential sellers.