There are literally thousands of dealers, auctions, private sellers, coin clubs and others all keen to sell you their gold coins. Most of them are genuine and bona fide.
Most people buy from dealers, mints or from auctions, eBay being easily the most popular.
When buying gold coins it is wise to keep the following in mind.
Ensure you buy from a reputable dealer. Ask questions such as 'How long has the dealer been in business?', 'What is their fixed address?', 'Do they take credit card?', 'Will they accept a return if the coin is not as described?' and 'What are their terms and conditions?'. Questions like that.
If buying from an auction you need to ensure the auction seller is bona fide. On eBay you can establish this by checking the feedback of the seller. Is the seller a power seller or at least someone who has a history of selling? If so, they are likely to be keen on repeat business and so will offer good service. Check their feedback rating including the percentage of positive to negative and read what other buyers had to say about their service. Anyone with a feedback percentage of 90% or less one should be wary of.
Other auctions may have similar systems to ensure that the seller is bona fide, such as seller registration requiring sellers to show some credentials to demonstrate who they are. If you are a serious buyer of rare gold coins then auction houses such as Sotherbys is a good start. One can get the nearest from the yellow pages and ring to see if there are any auctions pending with gold coins being offered.
Sometimes you might buy from a private individual. Here it is a matter of faith and trust. Do you know the person? Often friends and relations can be good because, at least, you know who they are. Perhaps you might like to work through an escrow company? Here the escrow company accepts the funds from you the buyer and the goods from the seller or some variation of that and ensures that all parties are satisfied before completing the exchange. They usually charge a fee for this service. Don't pick an escrow company the seller suggests. It might be their own!
Mostly, when you are buying gold coins it is a matter of due diligence and common sense to ensure that you know your buyer and even, for the buyer, to know the seller.
Where to buy gold coins then becomes a matter of being able to select the best price for the coins you want.