Wander around the floor with a magnet, and you should be able to grab it. Failing that, go to a hardware store. If it's a big enough store, they will have a screw that small. Failing that, go to a computer technician, preferably an independent Apple Authorized Service Technician, and if he's anything like our tech at work, he'll just give you one.
Apple Stores, while nice to look at, are not exceptional for answering questions, or dealing with odd technical queries. The primary reason for this is that they deal solely with Macs, where a guy who deals with PCs and Macs is more likely to have picked up some more indepth knowledge about the electronics. For example, when a slot loading optical drive fails, most Apple employees will suggest you replace the drive. However, this is ignoring the fact that these drives can be expensive (especially if it's a laptop drive), and a pain to install. My tech happened to mention to me that 99 times out of 100, the drive fails because a little piece of felt has started to peel away from the bottom of the drive, jamming the disc. So, you just need to slice it away with a knife. I really really dislike Apple Stores as a general rule. The guys there tend to have far less technical knowledge than I would like in a computer sales assistant.
. I was wondering if anyone knows where I could pick up a replacement for the 1.5 mil flat head phillips type screws that the pb uses ?