If you have an existing account or "entitlement" from a past purchase, you can try searching the Broadcom Support Portal. However, older versions are frequently hidden or restricted to those with active support contracts.
Once you have secured the ISO, the installation process generally follows these steps: vmware esxi 5.5 iso download
If you are planning to deploy ESXi 5.5 today, you must be aware of its support status: September 19, 2018. End of Technical Guidance: September 19, 2020. If you have an existing account or "entitlement"