International Gate 2 just before the immigration counters. Open 24 hours but not always manned. Tell an immigration officer that you need a re-entry stamp and they’ll get someone to do it for you.
If you have the photo and forms filled it’s the standard price of 1,
*****
,800. If you don’t then they add on an admin charge of 200 baht and you only need your passport.
Proof of money from abroad would need your embassy to issue an income affidavit. Not so many do that these days, especially US, UK, Australia and Canada - perhaps others.
Otherwise it’s banked money in a Thai bank account for the first year at least.
If you’re from one of the 93 countries allowed to enter visa exempt - that is with no visa, just a free entry stamp - then you’d get 60 days on entry.
There is no free visa for 30 days.
Bear in mind that the day you enter is day 1, so you will be stamped in for 59 further days.
Do the math and work out if you need an extension and if you don’t get one that you’d be happy with an overstay stamp and possible fine for the one day over.