Go to 275C Pham Ngu Lao St. D1 with your passport and a business card from your hotel along with your room number and they will have them delivered to your hotel the next day for 50,000dong per ticket, which means you will need to be in HCMC for a few days before you leave for Nha Trang. If you aren't then you should go straight to the "Ga Sai Gon" and take a ticket and wait an hour for your number to appear on the computer screen, so you need to sit up the front and pay diligent attention, because unless you speak Vietnamese and can count in Vietnamese you will miss your number and have to take another and wait another hour!!!