i would like to buy a GPS that I can plug into a rental car when i visit the USA, does anyone know if I can get one on samui or have any recommendation of a good brand I can buy in Thailand?

Tesco maybe

Samsung Galaxy Tab (of any size, or Galaxy Note if you want smaller), comes with free navigation software in Thailand (SpeedNavi)

Sold in almost every computer and telephone shop in Thailand, works very well.

The main thing you are going to need is the maps if you are going to use in usa so you will need to download unlocked maps from a web site.. A Garmin Nuvi 40 is cheap go to Gadgetrend web site and can buy for 4,000bt with Thai maps loaded they will send to you via ems.

Probably cheaper to buy it in the US. Also then at least it will have the right map preloaded. If you need to have it by the time you pick up your rental car at the airport; At most airports you can buy in the taxfree area, even when you are arriving (from abroad). Although for the US that would probably not be much of a difference as most states have no VAT or salestax.

Sirisin has Garmin GPS but you will need to download US maps.