Export to PNG or GIF.
Animate is not really an ideal program for producing print graphics.
Try Illustrator or Photoshop.
Use File/Export/Export Image or File/Export/Export Image (Legacy), and in either case choose PNG. If you're using the new image exporter make sure the transparency box is checked. If you're using the legacy one, make sure to select 32 bit.
In either case you might think about saving the image at a larger size than your stage size. Find out what DPI the teeshirt people print at, and multiply that by the size the picture needs to be on the shirt. For example, if they print at 150 DPI and the picture is to be 10 inches tall, export the image as 1500 pixels tall. You could export a lot bigger too, if you're sure they can scale the image correctly.