Mark Bittman says the best caviar comes from the Caspian Sea and, especially, Iran. However, Mark also says "caviar is to Iran what cigars are to Cuba - we can't get it!" Thus, we need to look to Russia for a quality Sevruga. Expect to pay about $20.00 per ounce. Alternatively, seek out American Hackleback at approximately $10.00 an ounce.
Mark recommends buying caviar in person if at all possible. If not, he likes to order from Browne Trading Company in Portland, Maine. You can reach them at 800-944-7848.