Options to learn  Khmer are all around. There are many children on the street selling books, this is probably the most difficult route to take. You can pick up more interactive software of book/software combos which will make things a bit easier. You can also find a local tutor or sign up for a course with some where like the International School of Phnom Penh or the Khmer School for expats and travelers.

Some free resources are available online, there is a Khmer language course provided by FSI. You can also Google for a download called Colloquial Cambodian.