Hi Ravi,
Is there any specific reason for an On-Prim training environment? given that no new licenses of On-Prim solution is being sold by SAP/Ariba Sales Team. For a corporate, the only way to get the installable for On-Prim solution is to get it through an existing on-prim customer account.
If your organization has partnered with Ariba, you may check if there is a demo on-demand (cloud) site available for you to use for training purpose but as you might be already aware there are limitations on customizations in the on-demand world so it will surely limit your L&D capabilities on core customizations.
If this is for an existing on-prim customer, you may want to check with them for access to a their Ariba installed Sandbox/Dev environment or create a new one if they will.
Regards
Vilas