how can i buy the GSM model A1429 in the us apple website?

The a1429 is the CDMA version. The a1428 is the GSM version.

Apple's website seems to indicate that there are two flavors of the a1429 model; one GSM and one CDMA. This is in addition to the GSM model a1428.

• GSM model A1428*: UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz); LTE (Bands 4 and 17)
• CDMA model A1429*: CDMA EV-DO Rev. A and Rev. B (800, 1900, 2100 MHz); UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz); LTE (Bands 1, 3, 5, 13, 25)
• GSM model A1429*: UMTS/HSPA+/DC-HSDPA (850, 900, 1900, 2100 MHz); GSM/EDGE (850, 900, 1800, 1900 MHz); LTE (Bands 1, 3, 5)

http://www.apple.com/iphone/specs.html

The gsm a1429 is not available in the US.

You can't get the A1429 form the online store, but you can get the A1429 that works in Europe from a physical store: https://discussions.apple.com/thread/4761497

The Verizon iPhone 5 is neither advertised, nor sold as an "unlocked" phone. The SIM slot is not locked down due to FCC requirements placed on Verizon in exchange for winning a spectrum auction. There are known issues when trying to use the Verizon iPhone 5 on a number of foreign networks.

Can you point me to those known issues? I have gone through 6 networks in Europe with 0 issues.

I'm not going to plod back through months of postings to satisfy your curiosity. You can search these forums for yourself. If it's working for you, that's nice. Plenty of people are using Verizon iPhone 5's on foreign networks. Others have issues ranging from problems with SMS to incompatible data networks.

To avoid issues, people should just buy a phone intended for use in their own region of the world.