We'll need a bit more information on the device and part. If you are looking for a replacement part you might type in the model number of your heater (into google) and add replacement relay. If you already did that you might try looking at Digikey or mouser. Both websites have a good selection and the ability to filter their selections to narrow down to the part you are looking for.
If you don't know where to begin with your filtering, check your device and the part you are trying to replace and post all the information you can find and we may be able to help point you in the right direction. You can also link a photo of the device and any text on the device. I work for an electronics manufacturer by day so I might recognize a brand mark or understand better the text on the part to determine the model number and help you that way.