All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The principle ruling is that one has to use water for purification purposes to pray. One is not allowed to use dust instead of water unless there is a Sharee‘ah-approved excuse to abandon Wudhoo’ (i.e. ablution) and perform Tayammum in its place. According to some scholars, nearing the end of a prayer time is one of these excuses. Consequently, if looking for water or performing Wudhoo’ will cause you to miss the prayer, then you are permitted to perform Tayammum. Otherwise, you are not permitted.
As for performing the prayer in the sitting position while one is able to stand up, this is impermissible. Being in a non-Muslim country does not justify your action unless you most likely believe that it is harmful for you and there is no safe place to pray standing up.
We bring to your attention that if you are unable to observe the rituals of your religion in a non-Muslim country, then you are required to migrate to a country where you can do this. You are not permitted to stay in non-Muslim countries without necessity.