Which airport should I fly into when wanting to visit Topsail Island ?
Wilmington (ILM) would be closer. RDU is a bigger airport and may have a direct flight if ILM does not. It's about 2.5 hrs easy drive to Topsail from RDU.
Yes, look at Wilmington and especially Raleigh. More options with Raleigh. Where are you coming in from?
You could also look into Jacksonville NC. Topsail is between Wilmington and Jacksonville.