If you want to stay w/ a COTS solution the Network Policy Server in Windows Server 2008 can function as a RADIUS proxy (though I have no experience actually using it for this). That may be a little more than you want to spend on a box for this purpose, but that's an option. A Microsoft VPN between the "network B" host and "network A" is a viable option for a VPN between the two.
On the open source front, the FreeRADIUS project can act as a RADIUS proxy. You could use something like OpenVPN to do a VPN between a machine on "network B" running FreeRADIUS in proxy mode and a host (even the Windwos Server 2003 machine hosting RADIUS, if you wanted) on "network A". That's more a "you get to build it yourself" solution, but if you've got an excess of time and a deficit of money that might also be a way to go.