George,

As you can imagine , there were several 4th Texas flags. Some of them have survived; in fact, I'm almost positive that one of them is on display right now at the Houston Museum of Fine Arts . The exhibition called " The Flags of Texas " runs through April 28th. I've been three times and believe me, the trip is worth it. Plus , in this era of political correctness ( PC) and heightened sensitivity to every ethnic and cultural group's feelings , I'm not sure those of us in middle age or beyond , will ever see some of these original Confederate flags again , in public. I think they ( museum) had a hard enough time even exhibiting the flags they did put out. So, if you can , go now. If not, go to the museum's web site and you will notice a book for sale about the wonderful Texas flags. My strong recommendation is to purchase the book.
Now , on to your next question about where you can purchase the good old Colonel's ( Simpson) works on The Texas Brigade. I would suggest you try some of the book sellers that specialize in the Civil War ( CW/ WBTS/etc.) One of these should have the late Col Simpson's work; Guidon Books in Arizona , Olde Soldier Books in Maryland; Broadfoot Publishing in North Carolina and C.C. Thompson. They all have web sites that can be found with a basic search engine. As you may know , there's tons of information written about the 4th Texas and "Hood's Texas Brigade" ( that is, the Texas Brigade that was with the Army of Northern Virginia ). There several other "Texas Brigades" throughout the various theaters of operations, e.g. Granbury's , Walker's , etc. However, "Hood's Texas Brigade " is who you are looking for. If you have difficulty finding these book shops,contact me off line and I'll get you pointed in the right direction. There is also a bookstores in Kerrville, TX ( and maybe Fredericksburg, TX, too)where Simpson's works can be found. Sorry, I don't fully trust Amazon , ebay ,et al for books anymore .
Good Hunting, Craig