IP Spoofing across different routers and across IPS's is not possible.
You can very easily do IP Spoofing within a WLAN, particularly easily if you're doing a MITM attack and using tools like ettercap filters. But going outside of the WLAN? Nope. Routers know what IPs are within each area and if an IP that is invalid in an area claims to come from it, it will kill the packet.
