A good place for small quantities of fabric fasteners is your local sail loft or boat cover shop. Marine supply stores usually don't have much variety.
On-line my favorite places for snap supplies are
Sailrite and
McMaster Carr.
Sailrite has the biggest selection under "Hardware" on their site and reasonable prices for the installation tools. They have a
PDF file that lists all the options in one nice place. Some are shown below.

If you have a solid surface around the vent or skylight to mount these to button snaps to they should work fine. Unfortunately the 3/16" ceiling covering, with foam behind it probably won't hold over the long term.
The following alternate type snaps hold really well when closed, but take very little force to open them vs button snaps.



One thing to consider above the bed area is, the twist lock type would hurt less than the other two if you don't have have much headroom and hit it with your head :)