If the federal government outlawed the selling of multiple round magazines, they wouldn't be sold in this country so you couldn't get one even if you lived in Texas - there wouldn't be any sold any more. Maybe, or not, you could find one a private person had and purchase that but that would be the only way to get one.
As laws have already been passed, if you look on websites of companies that sell these multiple round magazines, you will see they list the states and cities where they can't sell you a magazine, so you can't order one if you live in those restricted states/cities.
Wouldn’t make any difference if I already owned them. The same goes for ammo and guns. Another reason that you see the increased sales now and a lot of the sales you do not see as they are private sales. Best not to wait and then whine “I wish I would have....”.