Looks like you can get them online for Cambodia:
http://wikitravel.org/en/Cambodia#Get_inAlternatively, citizens of most nations can now apply for an e-Visa [3] online. The cost is US$25 (US$20 + US$5 processing charge) instead of the normal US$20, and you get the visa by e-mail in 3 business days. For the e-visa you will need one photograph of yourself. You can scan your passport photo (into .jpg format, please!) or take a passport photograph of yourself with a digital camera.
And for Vietnam, you should probably get it from the embassy in London:
http://wikitravel.org/en/Vietnam#Get_inIf purchasing your visa from the Vietnamese embassy in London, a 30-day single entry visa will cost you £38, plus £8 postage if you need it posted back to you (oddly, this information is not available on the embassy's own website).