The amount of iodine in most foods is pretty low (and some can be lost during the cooking process) which is why most salts these days are supplemented with it. In addition to this, cows are fed feeds that are supplemented with iodine in order to increase the concentration of it in their milk and thus dairy products. In general the major food sources are of marine origin e.g. sea vegetables, fish and shellfish.
As important as it is to get enough iodine during your pregnancy, it's also important not to go overboard. The recommended daily intake during pregnancy is 220 micrograms. It doesn't take a lot to meet this requirement. This site (ods.od.nih.gov/factsheets/iodine- ...) has a good list of foods and their iodine content. As an example, a serving of yoghurt, a 1/4 teaspoon of salt each day and a cup of milk together will give you ~200mcg of iodine.
Current USDA recommendations are that we can have approximately 1 teaspoon of salt per day (healthysparx.com/questions/4857/w ...). If you are making your own food then you can control this and ensure the salt that you use is iodised - in which case, you'll be obtaining your entire iodine requirements within that one teaspoon.