Think about resale. Gold bars are the cheapest. Gold American Eagles command a premium when you buy, but also get you more when you sell. Canada Maple Leafs and Krugerrands are also easy to sell.
The coins are also more available, making it easier to compare prices. Larger coins are cheaper per ounce of gold than smaller ones.
APMEX.com is one place to check. Tulving.com is cheaper but has very large minimum purchases. To avoid shipping/insurance cost, you can buy locally from a coin shop. You'll pay a bit more than APMEX but some feel there is a benefit to being able to pay in cash with no records kept.
For example, today APMEX is charging $95 over melt value for 1 ounce Gold American Eagles, and pays $59 over melt value if you want to sell to them.