Ah screws.... thou are a fickle thing....
Your only real choices (in my opinion anyway) are as follows:
1- Contact the manufacturer, retailer or distributor and see if they have any parts available, or suggestions on where parts may be located. This is perhaps your best bet.
2- If option #1 fails, you can try to identify the size, pattern etc of the screw using something like
THIS INFO from a place like Bolt Depot. From there, you can try contacting hardware stores, fastener distributors (Fastenal is the one I use most), eBay or other sources to see if you can find something close enough that will work. If you are lucky, you may have a knowledgeable enough guy at your local Home Depot (or equivalent) that can measure your screw for you and provide proper info for you, but this is really a bit of a desperation option and #1 is a much better bet.
Sorry I can't be of more help, but this is a problem I have run into on a number of occasions, and it is often easier to find another solution (JB Weld or epoxy resin perhaps?) than it is to find the right screw if the manufacturer, distributor or retailer can't help you.
Def