Getting a US SIM card for your smartphone while in the US will make your travels much easier by accessing the internet on the go and using Google Maps. Car rental companies offer navigation systems, but they can get costly - especially for longer trips since they charge you per day. A better option is to purchase a SIM card for your unlocked GSM smartphone. Be sure to check if your phone is unlocked with your provider back home first. If you do not own a GSM phone, you can always purchase an inexpensive phone here along with your SIM card.
Two carriers provide service for unlocked GSM phones here in the US - T-Mobile and AT&T. You can purchase a SIM card directly from either carrier at a nearby retail location listed below, or stores like Target, Walmart or Best Buy. One of the most affordable (but slower) options is purchasing a Straight Talk SIM card at Walmart starting at just $30. Just note that Walmart's are located in the suburbs, so if you do not have a car, it will be best to visit one of the nearby T-Mobile stores downtown.
T-Mobile (0.8 miles away)
Fig at 7th (2629)
725 S. Figueroa St., Suite 139
Los Angeles, 90017
Click here to see pricing - start at $50
AT&T (3 miles away)
3764 Wilshire Blvd.
Los Angeles, 90010
Click here to see pricing - start at $60
AT&T reportedly offers the fastest speeds on their LTE network if you have a compatible phone, but using T-Mobile or Straight Talk will not be too bad. Straight Talk actually runs on the AT&T network, but you are limited to about 2G speeds. Basically, you get what you pay for.