It’s great isn’t it , all this paperwork to purchase a motor bike
Then 25% of the Thai population ride around with no helmet , no driving licence , no insurance , no Tax paid and all the family riding , husband , wife and two kids
any Bank can supply you with Foreign Currency , but the rates may not be good, I would suggest you get enough to last you a week $600 dollars worth
Then bring dollars with you to exchange for BHT
All the Tourist Places have Multiple Bureaux de Change Kiosks as do the Major Cities usually in the Shopping Malls
Avoid using your credit card in an ATM
the rates are appalling and they will charge a fee
If you are coming long term a Thai Bank account is a good idea and then use Wise Money Transfer app to transfer money from your USA Bank to Thai Bank or any one in Thailand with a Thai bank account