http://www.usagold.com/ (you can buy from these guys and also ask questions in the forum)
http://www.southwesterngold.com (I use these guys. Ask for Phil. They will buy or sell.)
http://www.gold-eagle.com (check out the forum for information)
http://www.lemetropolecafe.com (try a two week subscription for free)
http://www.goldensextant.com/ (nice financial articles)
http://www.ebay.com (search term = gold or silver coin)
http://www.kitco.com (check out the live quote page and the forum)
That is just for starters.