As you get more and more names, and as you talk to those people, you will discover there are some who are extremely helpful and some who are not. You can thank them politely, send a short written note and call it quits. They simply do not meet your needs for a member of your network. Then place the person's name at the end of your list. Most associations have reduced membership fees, and some require no fees at all until the member is employed in the profession.
Most associations produce membership address lists which are usually free to members.
Every person on the list is a potential contact. Do not worry about it. You may want to make another call, especially if it has been several months since you last talked. Your call could be fortuitous for both of you. Working on the hot contacts without letting the rest get cold means devoting some time and energy to this process.