Any electronics or computer store shoul have a good selection of wireless routers or access points. A wireless access point would be all you need to plug into your existing switch, but an access point would likely cost about the same as an entire wireless router. If you bought another router, you wouldn't necessarily have to use it as a router. Just plug in one of the lan ports to one of the lan ports on your existing router or swith and configure the wireless DHCP settings to tell the wireless computer to use the other router as the gateway.
Jeff
Jeff