Answer
The performing of two Rakaats Nafl Salaat with the intention of
Istikhaara is desirable, it is not a prerequisite for the validity of the
Istikhaara. It would suffice to perform Wudhu and recite the Dua of
Istikhaara on her Musalla (prayer mat) before sleeping without performing
the Salaat for Istikhaara.
From Al Mahmood 10