If it's just the IP you need to spoof, work below the http layer to stay on the safe side. This translates in ordinary terms to: vpn, socks (use version 4a not to leak the DNS queries), basically. Http proxies are NOT meant by design to and they're NOT gonna work at tcp/ip level unless you use http proxies that work thru the CONNECT method that's NOT -generally- the standard way http proxies you buy (level 1,2,3,n) work. These latter simply relay the http requests, NOT the entire tcp packets that carry the http requests, so they're prone to a number of leaks if a different, undetectable IP is a mandatory requirement.