I'm able to code one for you in php. Please add me in MSN so we can discuss the details: c-sh[at]live dot com
What carriers do you need to provide? You can see an example of an old SMS script I wrote: http://www.empathogen.net/phpmysms/
I have already written something like that for about 10 clients. If you are interested, add me to your msn. Regards, Nitsan