Can I print a Texas flag on a t-shirt and sell it? If not, how can I do it legally... who owns the copyright on the Texas flag?

Yes, generally flags aren't copyrightable and governments can't own copyrights or trademarks, although there are some exceptions to that rule. You could design your own version of the Texas state flag and copyright that (at least your own creative contributions) as well. Still images from movies are different, as those movies and images taken from them are owned by the producer/studio that created them. Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

Yes, you can place the flag of the State of Texas on clothing and lawfully sell the clothing. No one, including the State of Texas, owns the exclusive right to make, sell, or display the Texas state flag. It's a crime under the Texas penal code if a person "damages, defaces, mutilates, or burns the flag of ... the State of Texas." However, flags displayed on clothing do not qualify for protection under this statute. Odd. In short, you're free to sell flag-emblazened clothes.