As badams said above, the best liquor store in Houston is the Spec's Warehouse Liquor Store located close to downtown. If you would like to mapquest the location the address is:
2410 Smith Street
Houston, TX 77006
The web site is: http://www.specsonline.com
Go to "Browse the Store" on the left-hand side of the main page to see the available selections. The selections are a little bit slim at this time, following the holiday season buying, but one of the store managers told me last week that they should be re-stocking over the next few weeks. There are about 20 satellite Spec's stores spread throughout Houston, but these are generally much smaller than the main store.
Another store that is good, but not the size of Spec's, is Richard's Liquor Store, with the main store at:
2124 South Shepherd Drive, Houston, TX 77019
As far as what malts are available which you can only get in the USA, I'm really not very knowledgeable about this. Looking at your web site, I'm not sure if you'll find any SMSW's only available in USA in Houston. Spec's has a private/independent bottling of Macallan in 12, 15, and 18 yo expressions under the label of Ste. Michelle. These are OK but not great. Spec's does have an excellent selection of American whiskeys if you're interested in any of these malts.
If you would like any more info, please contact me. Please let me know if you need any assistance while in Houston and I will certainly try to help. A word of warning if you have not been to Houston before: Houston has very poor public transportation.