Hey Ravi,
I am certainly not recommending you to use existing customer's instance/environment without their permission. Yes you can get a downloadable version of upstream and downstream installable on connect site for only the purchased product line. If you go to download software after logging into connect.ariba.com, you will not see the on-prim product line if you (as an organization) do not own a on-prim solution. Every customer of Ariba gets a connect site access and can download all Ariba propriety software linked to their purchased license.
Regards
Vilas