The cheapest way is to take £,€ or $ and change it, there are fx shops every 50 metres at least 4 around Nana Plaza, just check their rates and use the best, they seem better than the one at the airport so just change the minimum
Next best is to use a currency card just google it , use the rate from the card, however the machines charge yo 200 baht per transaction minimum
Then there is your own bank or credits cards, usually commission or fx transaction fee and rates very variable and again 200 baht per transaction minimum also some credit cards charge interest immediately on a cash withdrawal
That’s my experience anyway