Unless I misunderstand the question, an .ai file is fine. I am assuming you want to print digital transfers from an inkjet printer with pigment ink. You would print from Illustrator onto your transfer paper. What is more important is a correct icc color profile made for your specific printer, ink and media. Unless you have an icc color profile, your colors may or may not be very accurate. The vendor you buy your supplies from may have color profiles for the products they sell. A calibrated monitor helps in the equation as well.