While i can not think of a true router per se for this, you could go another route.
Take a PC (could even be as low as a P II) with a wireless card int it, put in at least one Nic (or more depending on how your internal network is set up), install Linux on it, and do the routing from the linux ip tables.
A real easy way to do this, would just put smoothwall on the PC (Linux based firewall), and you could use that to configure the IP tables, plus add security for your network!
Hope this helps