Assuming you mean a site built home, I don't believe you'll find anything near Oak Island for under $100k. However there are a few under $120k in Southport and Boiling Spring Lakes. My best advise is to seek out a local Realtor to help you. If you'd like to e-mail me I'd be glad to make a couple of recommendations. In the meantime, here's the link to the Brunswick County Board of Realtors where you can search based on your criteria.
I am going to give you a website for a company I have used in the past that specializes in retirement and vacation homes. They will have the best information on the coast of NC. The email is firstname.lastname@example.org the website is below.