A couple more good hot-weather cultivars: Jetsetter, Creole.
Skyfire sells a mix of cultivars that have done better than
most in mid-summer heat in Kansas (it says 25-30 seeds,
and I don't know whether they are sorted by cultivar).
Their catalog also has notes on how individual cultivars
that they sell seeds for did in the heat:
http://skyfiregardenseeds.com/#Tomatoes
Beyond a certain air temperature, that varies with cultivar,
there just is not a lot you can do, because pollen becomes unfertile (and to some extent ova in the flowers, too). One
trick is to go around early in the morning, before it heats
up, and shake the plants. Some flowers will get fertilized
by pollen that developed the night before and has not
been exposed to the heat of day yet.