What exactly did you have in mind for your flower? Did you know what kind you wanted?
You should be able to get a hair clip from wal-mart or any arts and crafts store (they have TONS of different styles to choose from -pick something descrete that won't stick out obnixiously under a flower but will be strong enough to hold your hair type.)
Once you go to your arts and crafts store and pick out the flower that you want, take some wire cutters and snip the stem at the base of the flower...if you want to secure that the flower is going to firmly stay in place on the clip my suggestion would be to get a little chunk of floral clay and put that on the clip...then put a line of hot glue on top of that and add your flowers. Put some pressure on it for a few seconds until it stays put....let it dry for a few hours and your good to go.
A suggestion on the glue gun....I always use an industrial strength glue gun when I'm designing....keep in mind that the hotter the glue, the better the hold will be so it's nice to get a high heat glue gun if you can.
Those cheap small glue guns are useless when it comes to really getting a good bond out of them....the higher the heat the better- so don't skimp on the glue gun
If you can't fix a clip for yourself I'd be more than happy to help you out if you wanna pm me.