Sandstorm has several races in it that gain Heat Endurance as a bonus feat:
- Badlands dwarves
- Scablands half-orcs
Since these are racial bonus feats, you do not need to worry about meeting the Fortitude requirement. Any one of them except the half-orc works fine for a wizard.
However, taking one of these races has a problem with Southern Magician—none of them are human. Alternate Source Spell becomes an option, but that’s more costly to take (and if you were dipping a divine class, you could just dip cleric and get the Fortitude anyway). Interestingly, the half-orc is better here: per Races of Destiny, a half-human can take things specific to humans (depending on your DM, you may need to take the Human Heritage feat from the same source first, though).
The −2 Int is a problem, though. And since half-orcs never get any love, I’m not sure there’s any way to make a half-orc without the Intelligence penalty (still looking). A Kingdoms of Kalamar supplement, Fury in the Wastes: The Orcs of Tellene, includes black orcs that do not have an Intelligence penalty, but no word on what black half-orcs might be like (or whether they’d be compatible with scablands half-orcs). Likewise, Dragon vol. 339 updates the Spelljammer races to 3.5 rules and includes the scro, orcs-in-space, and they have no Intelligence penalty, but there is no mention of half-scro.
If you want more cheese, these are all Humanoids. That means alter self will let you become one, gaining Heat Endurance. The rules for qualifying for prestige classes say you only need to meet the prerequisites to take the first level—if you can ensure you level-up to 4th while alter self is in effect, you are eligible, RAW, to take walker in the waste and then keep its class features and keep taking its levels after alter self wears off. Most DMs are unlikely to go for that, though.