There are various sources over the internet. You can look for individual offers and companies. Mints, merchants, and jewelers who operate in actual locations are also potential sources. For your own assurance, you can make a quick background check first before transacting. Find out if the company is legitimate enough and can be trusted. Make sure that the items they sell pass standards of purity and mass.
For gold coins, aside from having a passing purity percentage of at least 99.5% and a standard mass for gold items, their authenticity should be backed up in writing. The American Eagles gold coins particularly those which are released annually have accompanying official seals which assure a purchaser that the items are genuine.