This program you can download from everyone site in Internet. Try open google,
http://google.com/search?q=download+WebStressTester
Requirements:
1 GB RAM for each 100 threads
100 kbps for each thread minimum
10 kbps upload for each thread minimum

450 mbit/s attack maximum.
If site located on free hosting, 1 threads and 1 client = 100 kbps
If site located on simple hosting, 1 threads and 10 clients = 1000 kbps
If site located on standard hosting, 10 threads and 20 clients = 20 mbps
If site located on advanced hosting, 100 threads and 20 clients (200 mbps) + required

If site located on VDS hosting with fast internet connection (e.g. 1GB/s), your need start WebStressTester on your friends computers. You need 10+ friends for better attack.
You can create botnet and add WebStressTester to your botnet (with hiding window)
Read HowToDetermineHosting & HowToDetermineHostingPlan.

*mark: bytes in second