The Screws at the bottom of a MacBook Air

im not too sure if anybody had this problem before.
but now,
one of my screw are missing... what should i do ?
where can i get one back ?

moreover, i think when i go fix my mac with the AASP in Canada,
they somehow make my air screws easy to lose, may be they use a wrong screw driver,
or, they tried to put some other screws in.. i dont know.
and i didint realize it at the beginning,
but after a couple weeks, i realize that the screws are losing, so i tried to use a screwdriver to
make it more tight, at that time, i realize the screws are easy to lose...

so what should i do and what can i do .
its already been like half year already,

i now use some tape to tape on the screws incase, another one lost.
:'(
please help me,
i have no idea what i should do now and
where i can get the screws