What is the process for obtaining a re-entry permit for a single entry non-immigrant visa in Thailand?

Advice re: re-entry permit (single entry non-immigrant visa).

(*edit* so sorry, I forgot to mention I am based in Chiang Mai)

I realise a common question, but this one is a little outside of the usual, so hoping for advice please.

I plan to ride down to O'Smach Cambodia border and enter into Cambodia with my motorbike. I am currently on a single entry non-immigrant visa.

Do smaller border crossings have re-entry permits or is that too much to hope for?

My guess is it is not worth taking the gamble and instead to get one prior to heading down there...

So, if I am best getting one before-hand, I should go to immigration near the airport, right ? (I can look this info up, not so worried about that part).

When I get the re-entry permit can I leave the date of exit open? Is it like a use within one month thing that expires after a month? ..because I am not exactly sure of my date of entry at O'Smach.

Thanks all. !