Spec's Deli downtown. I didn't scroll the whole list but spotted a couple of ghost pepper sauces right toward the top. It may help to know a brand name since the names may not always mention the pepper involved.
Spec's is always a good place to check for specialty food items but note that just because it's listed doesn't mean they'll have any in stock.
You can buy Melinda's ghost pepper sauce in a lot of normal grocery stores. I don't really care for really vinegary sauces though. I don't know about stores in the Houston area, but www.peppers.com has a massive selection of hot sauces, they're probably no more expensive and maybe cheaper after shipping than what you'll pay in a retail store, especially if you buy cases. I like Heartbreaking Dawn's 1498 made with Trinidad scorpions and scotch bonnets. Its heat isn't too extreme but the flavor is great. They also have 1498 Cauterizer which is the same recipe but uses only Trinidad scorpions, no scotch bonnets.
There aren't going to be a whole lot of Moruga sauces because it just this year tested so high. Not many people were growing it in large quantities before this year. CaJohn makes a Moruga sauce, but it's a vinegary/acidic one. I've grown all of these peppers for a few years. IMO, the Moruga and 7 Pod brain strain (some think these are the same pepper, but my plants always look markedly different) are the hottest, followed by Trinidad scorpion and 7 pod Douglah,, then 7 Pod, and then bhut jolokia/bih jolokia/dorset naga. So there are more options than just those two as well.