I originally made a comment on some answer, but I think that the person asking the question is confusing anonymity with IP spoofing, which are different beasts altogether, so I'll go into a bit more detail.
In a typical environment, you can not spoof an IP address for an HTTP request, and this is because an HTTP request is running over the TCP stack. Pedantic stuff aside, the typical TCP stack consists of packets, with headers, and a 3-way handshake.
HTTP requires a full on TCP handshake, followed by the HTTP protocol "stuff" (the actual communication of HTTP request/response). One answer by @Edvinauskas mentioned nmap, and what they were referring to is not at all related to HTTP requests (or anything requiring a full handshake) but rather port scanning which, simplistically, can infer that if 2 parts of a handshake complete, probably that port is maybe open (SYN scan, used by other tools like zmap, masscan, etc.).
But no, that wont work in this case. The HTTP request will never go through.
In the question, it seems that what is being asked about is sequential requests, so you want it to look like basically every time you send an HTTP request to some server, it looks like you are coming from a different IP address.
This is handled typically in the following way:
Use a lot of proxy servers, not just one
There are edge cases to this, but as edge cases go, they probably do not apply to your question (e.g. if you are sending mass spam, or masquerading as something you are not).