Cambodian visas are available at the border. Your options between Phnom Penh and Ho Chi Minh City via Moc Bai includes the following:

1. The direct Phnom Penh Sorya Transport bus is a large, air-conditioned bus, which departs Ho Chi Minh everyday at at 6:00am. It’s $12 per person and you don’t change buses at border.
2. Daily minivans from Capitol Guesthouse and Neak Krorhorm Travel for $12. Bus to the border, check through immigration, walk across the border and meet a prearranged bus on the other side.
3. The cheapest option are local taxis which can be arranged from Ho Chi Minh. At the border, check through immigration and walk across the border. To PP from here, shared taxi costs $5 for a cramped and harrowing ride or a private taxi for $35.