acceptance letter with no letter of financial support
How about,

Dear Prof,
I am an international student and need to apply for a visa to come to the University. I will miss the winter semester if I don't have the letter of financial support in 2 weeks.
Best regards, Nikki.

Be polite but straightforward and always explain why the deadline is real. Don't hesitate to chase people - it might be that the professor is simply too busy and he keeps forgetting things which are not urgent. Note also that his understanding of urgency can be different from yours.