Get yourself a plastic ATM/Debit card linked to your home account. Then draw down money in 10,000 or 20,000 Baht amounts.
Expect to pay B 150 - 200 per withdrawal as banks in LoS have found another easy way to gouge foreigners.
Generally speaking, ATM exchange rates (ER) are among the best available.
Personal Opinion: I would not have large amounts of foreign and Thai cash in my wallet or my hotel room. That is just fool-hardy.
However, if you have an at-home surplus of cash and it makes sense to use it, find a SuperRich currency exchange for the highest rates.
Withdrawing cash from your home account to exchange after arrival in LoS (a) is fool-hardy, and (b) does not give the best ER.
If you shop around in your home town, the ER for Baht may be beneficial and you have Baht in your pocket when you arrive.
All ER are available on the Internet.