Can an person holding an American passport in Thailand get a Multi entry visa for Cambodia? If so, then where and how?

In Cambodia, any business visa of 6 months or more is automtically multiple entry.

When you enter the country, at an airport or land crossing, you first apply for a business visa. You do this by stating BUSINESS (instead of TOURISM) on the application form as the purpose of your trip. This visa will cost you $ 25 instead of $ 20, and you don't have to provide any further justification. This visa is single entry only and valid one month.

Before this visa expires, you apply for an extension of 6 or 12 months - a 3 months extension would be single entry only. You can do this at any travel agency at a cost of a little less than $ 150 or $ 300, respectively. You can also apply directly at the Immigration Office opposite to the Phnom Penh Airport which may save you som time and money.

That's it.