Soluable, from what I have read, is best for constipation because it acts like a lubricant, but all fiber contains both soluable and insoluable. But supplements (and fruits and vegetables too) tend to have more of one kind and less of the other. Most fruits, for instance, have more soluable fiber, whereas tough veggies like bell peppers and carrots are more insoluable.
You can find fiber supplements in the bowel-products section of any pharmacy or a grocery store with a pharmacy. If you look at the label, it will tell you how much of the product is soluable and how much is insoluable. If you're not used to taking fiber, you might want to start with just half of the recommended doseage, as it can take a couple of weeks to get your system adjusted so that you don't have excess gas. After a couple of weeks, up your dose to the recommended amount.