Saying du’aa’ al-istikhaarah without praying

Istikhaarah means asking
Allaah to help you choose the better of two things and to divert the worse
of them. Praying two rak’ahs beforehand is only prescribed before the du’aa’
to make it more likely to receive a response, and to help bring about proper
presence of mind and make the du’aa’ more likely to be accepted. 

Ibn Abi Jamrah said, as
narrated in Fath al-Baari (11/186): 

The wisdom behind putting
the prayer before the du’aa’ is that the aim of istikhaarah is combine the
best of this world and the Hereafter. So he needs an introduction before
presenting his case before Allaah, as it were. And there is nothing better
for that than prayer, because it involves venerating Allaah, praising Him
and expressing one’s need for Him now and always. End quote. 

The istikhaarah that the
Prophet (peace and blessings of Allaah be upon him) taught us includes
praying two rak’ahs then reciting the du’aa’ after that. 

If a Muslim cannot offer
the prayer (two rak’ahs) for some reason, there is nothing wrong with
calling upon Allaah and asking Him to make easy for him the better of the
two options, without offering the prayer. 

Al-Nawawi (may Allaah have
mercy on him) said in al-Adhkaar (p. 120): 

If he cannot offer the
prayer then he may recite the du’aa’ of istikhaarah. End quote. 

These words of al-Nawawi
were quoted by a number of scholars from different madhhabs, who quoted them
as evidence. 

See: Haashiyat Radd
al-Muhtaar (2/27); al-Fawaakih al-Dawaani (1/35); Sharh
Mukhtasar Khaleel by al-Kharashi (1/37); Asna’l-Mataalib (1/205). 

And Allaah knows best.