I'm in a hot zone also, I find the cherry tomatoes do much better than the larger varieties.

Here is a link that might be useful: DFW Gardener

Thanks!

Tomatoes can't set fruit when temps or humidity are too high.

When daily highs are above 90-95 and nighttime lows are either above 75 or below 55, pollen becomes unviable and fruit can't set.

The same is true of really high humidity, but I don't know the numbers.

Some varieties will set at higher temps than other varieties. Cherries are among those who tolerate heat better.

I don't know anything about your region, but if you aren't finding seedlings for sale at this time, it may be because the weather is unfavorable. You might ask your county's Cooperative Extension office when would be a good time to plant. Click on your state here, and you'll get the county list:
http://www.csrees.usda.gov/Extension/