Where can I book a budget bus ticket from Bangkok to Siem Reap for a visa application?

@Riaan *********

Similar to most things in Thailand, I think it’s a bit of luck on who you get and what they ask for 😕

As Wikitravel mentions:

Proof of onward transit :- long happily ignored by Thai immigration, has been known to be strictly applied in some instances (Indian passport holders beware). The requirement is for an international flight itinerary - NOT train, ferry, or other departure type. Airlines, who have to pay for your return flight if Thai immigration doesn't let you into the country, also check this and often will not let you board your flight for Thailand without it.) A print-out of an international e-ticket on a budget airline is sufficient to convince the enforcers, but those planning on continuing by land may have to get a little creative. Buying a fully refundable ticket and getting it refunded once in Thailand is also an option. Land crossings, on the other hand, are a very straightforward process and proof of onward journey is generally not required (Indian passport holders beware again... or anyone, if the border officials simply decide to uphold the bureaucracy).

@Bonnie ******

It's boarding your flight that's the problem not Thai Immigration, the airline wont accept that is your plan 😂

@Marc *******

you can try, I hope it will be accepted for the visa application. As a Thai bus actually never crosses the border to Cambodia (left hand-right hand)