Onelook.com

This website is extremely helpful if you are look for the proper way to spell a word, since it depends on what dictionary you're looking at. The great thing about Onelook is that it will tell you how many and which dictionaries use which spellings.

There are two different ways you can spell goodbye and it shows that "goodbye" is the most accepted one. Most often I will trust the Merriam-Webster or the American Heritage dictionaries.