This only needs to work on a single subnet and is not for malicious use.  
I have a load testing tool written in Python that basically blasts HTTP requests at a URL.  I need to run performance tests against an IP-based load balancer, so the requests must come from a range of IP's.  Most commercial performance tools provide this functionality, but I want to build it into my own.
The tool uses Python's urllib2 for transport.  Is it possible to send HTTP requests with spoofed IP addresses for the packets making up the request?
I suggest seeing if you can configure your load balancer to make it's decision based on the X-Forwarded-For header, rather than the source IP of the packet containing the HTTP request.  I know that most of the significant commercial load balancers have this capability.
If you can't do that, then I suggest that you probably need to configure a linux box with a whole heap of secondary IP's - don't bother configuring static routes on the LB, just make your linux box the default gateway of the LB device.
Spoofing the origination IP address of an HTTP request