I was recently asked when the IBM customised ISO for ESXi 6.0 would be available to download. However, since the sale of xSeries to Lenovo, any future ESXi customisations will be provided by Lenovo.
The Lenovo Custom Image for ESXi 6.0 is avaliable to download here.
ESXi 5.5u2: IBM Fix Central
Older ESXi versions are available here