Dear Sai,
There are 2 ways you can handle this
1) When you do plant to plant transfer , you will have two line items getting created . One will show the receiving plant and another will show the issue plant.
You can see the debit/credit indicator and based on that you can get the issue plant through a start routine.
2) In the receiving plant material document item check for the field valuation area. This always bring the valuated area from where the material is issued and normally this field will carry the issuing plant.
Regards
Gajesh