The Satsuma mandarin/orange is one of the most cold hardy varieties; it survives temperatures to the mid 20's; below that a light blanket thrown over the tree with a 100 watt light bulb inside, it will likely make it thru temps down to about 20. Good choice, easy to grow, productive and tasty. I would suspect you could find some in the Rio Grande Valley if you look around a bit.

Hi insteng, i have a place in Napa, Ca which gets down to the low twenties for a few hours for 3-12 nights in January (this past Jan was 3 weeks). I am not sure how that relates to your temps.

One night at 18F last year convinced me that i cannot get away with growing late season varieties. All the fruit needs to be off the trees by mid to late Dec. [I don't want to bother with any special protection, but probably will do so when temps threaten the mid/low teens].
Some endure the low temps better than others of course; here is my list of hardiest to least. (I have included planting location because that can add several degrees):

Satsumas (Okistu Wase) - excellent and took the lows like champs (north corner of house - coldest area)
Trovita, Washington Navel, Fukumoto Navel - also no issues (away from house with no protection).
Moro B Orange - some leaf burn on early fall flush, fruit excellent mid/late Dec. (against stucco wall, west facing)
Page Mandarin - Fukushu Kumquat, & Nippon Orangequat - no issues (and out in open - no pretection)
Oroblanco grapefruit - minimal damage, fruit took 24F ok
Bearss Lime - late fall flush destroyed, lower fruit ok after 24F
(planted against concrete patio and / or fence - that helps)
Eureka V. Pink Lemon - fall flush destroyed (against stucco south facing wall)
Lisbon Lemon - fall flush destroyed, fruit higher than 18" off ground destroyed (against fence and concrete patio corner)
Meyer Lemon - fall flush destroyed and some further damage, but now in strong spring flush (one against a south stucco wall, the other in open... the one out in open was clearly damaged more)
Eustis and Taveres Limequats - fall flush and all fruit destroyed [these are my least favorite perhaps because i haven't had them produce any usable fruit here yet] (cold unprotected location)

I hope that helps you!

Insteng, most nurseries have citrus right now. Check with Buchanan's in the Heights, or JRN Nursery on Harwin west of Wilcrest, they have a huge selection. If you're near Tomball, RCW on 249, I think, by the Beltway, they also have a lot of citrus. They can help you with citrus selections, too. Moro Blood Orange would be a good choice, too - they need more cold than Houston usually has to make the dark juice.

I found several trees this weekend at a local nursery. I will probably pick up a couple this coming week and try them at my farm. I figure if I get them planted this year it gives me a year head start on them producing. I just need to decide where to plant them. The one spot is near my barn but they would be exposed to a full north wind. I will probably look for a location where they have protection from the north wind. Luckily I have over 50 acres to play with so I have plenty of options.