Can you still buy schweppes Russian mixer?

Yes, in the UK (04.07.08).

The bottle is a 1 litre, light pink/clear colour with the same design cues as the Indian Tonic Water Lemon Zest. The actual name is Schweppes Russchian.

As for other countries, I don't know if it is available there or not.

Schweppes Russchian is only available in Europe, (in the UK it is generally marketed as Tonic Water with Russchian) and is still a best seller in Scandinavian countries (20/12/08).