I was looking over in Port-A last May and there were no vacancies. I don't think you would have a problem finding a place in Aransas Pass or Port O'Connor. I kept my boat over in POC until last month, a 40 foot stall cost me $70 per month.
I keep mine at squyires boat storage, its on 188 between aransas pass and rockport.
Its down the road from the big fisherman. the units are 95.00 a month. Its real convient for us coming from austin, the owner is a really nice older gentleman. they have been built in the last 2 years. He has onsite security and a coded gate and has a pressure washer there.