I know exactly what type of garage you mean, as we've got the same one. The best we've done is put bricks in front of the door, but that was only supposed to stop the wind from blowing the door open. The lock in the handle doesn't seem to work with our door, but I've had the same one in the past and they worked well as a lock. Does the one you've got not work properly?
I've got no idea how much they'd cost, but I'm sure you'd be able to get a standard garage door handle (with lock) like the one you've got now in Homebase or B&Q, or some place like that, as I think they're pretty common.
Or you might need a new garage door which would come with a working lock and key, but that might be a bit expensive for what you want.
In summary, I'm not much help, sorry.
