As the capital, Phnom Penh is serviced by buses from all corners of Cambodia. You can often book buses through hostels and hotels in Cambodia. Alternatively, I’ve found the easiest and cheapest way to book transport in advance is through Baolau*, a search engine that compares all travel options to find the best one for you.
You can also book tickets directly through VET Travel, Cambodia’s biggest bus company that services routes all across the country.
Getting the bus from Ho Chi Minh City to Phnom Penh
It used to be possible to take the bus from Ho Chi Minh City, Vietnam, directly to Phnom Penh, Cambodia. Due to COVID, this route has been temporarily suspended but I suspect it will return by the end of 2022. Normally these buses can be found on search sites, like Baolau*, and through accommodation in Vietnam.
Currently, to cross overland from Vietnam to Cambodia you need to take a bus to the border in Vietnam and then catch another bus on the Cambodian side to Phnom Penh. I haven’t personally done the journey like this, but I have heard many hostels can help arrange this for guests.