Short Answer: Zone 4, encounter Tauros, hit it with a rock then throw balls.
There are a few things to note when going for rarer Pokemon in the Safari Zone: they have low catch rates and low encounter rates, so a large degree of catching them depends on luck.
That being said, there are more or less optimal ways to find and catch Tauros and Chansey. The various areas of the Safari Zone offer different encounter rates for rare Pokemon. For Tauros, it is ideal to go to Zone 4 (where HM03 and Golden Teeth are found) and roam the grass south of the house and pond. Tauros has a 4 percent encounter rate, but for real-time estimation, I would run into Tauros at least once every 5 minutes or so.
Rather than running around, turn your character in place to avoid wasting steps. This will ensure you don't waste more time than you need to, and you'll use all of your balls catching the Pokemon you're after before needing to leave the Zone.
I have tried a few different approaches to catching rare Pokemon like Tauros, and it seems that the most relevant challenge is their low catch rate. Don't listen to the guys saying throwing only balls in the Safari Zone is the most efficient way to catch Pokemon; it's not. Tauros and Chansey have low catch rates, encounter rates, and run quickly, and while you can't influence their encounter rate (aside from being in the optimal zone), rocks heighten catch rates. In addition to that, if you use bait or chuck balls, you are more likely to waste balls and need to leave the Safari Zone sooner, wasting more time in the process. Tauros and Chansey may run after one hit from a rock, on the first turn, but if they don't, you're significantly more likely to catch them. Since they run relatively quickly anyway (I just ran into 10 tonight and they averaged approximately 2-3 turns) just hit them. I've had much more success this way.
I don't remember the optimal zone for catching Chansey, but likewise, rock it.