Note that the achievement means 500 chips cumulative, not 500 chips at one time. While this achievement is technically missable, there are numerous opportunities to easily get it. Not to mention unless you are exceedingly lucky you will need many more chips than 500 just to survive. The most natural way to attain this is to scavenge everywhere in the Town for chips. Each container usually has 8-15 chips in it (the Scrounger perk will significantly increase this amount), so over time you'll accumulate enough.
If you still haven't gotten it once you enter the Casino, you can simply gamble for the chips. Once you've turned the power on, the hologram table workers activate allowing you to play Blackjack or Roulette with your Sierra Madre chips. Simply gamble until you have the 500 chips (though I suggest farming for much longer so you can simply vend everything you'll need for the rest of the DLC). If you need help with earning chips through gambling, see The Courier Who Broke the Bank in the main achievement guide.
You can still get the achievement in the Mojave wasteland in one of two ways. First, you can make counterfeit chips if you received the Coin Operator perk from Christine during the DLC. Note the Coin Operator perk is learned from Christine, not given as her companion perk. You can use these counterfeits to buy goods from the vending machine in the abandoned BoS bunker. Next, sell back the goods you bought from the machine to receive real chips. Alternatively, you can simply bring items you find around the wasteland back to the machine, return them, and receive chips for it. The only way to truly miss this achievement is to not get enough chips during the DLC and not have found the return codes for at least one item. In other words, not likely.