Any recommendations for local firms rather than the large chains?

List where you're staying and others can give you a better idea on where to rent.

Sorry I'm based in Bangkok on Rachadapisek.

I used Thai Rentacar for a few years a long time ago. Good service, clean cars with easy pick up drop off or delivery near the airport or on Petchburi. Nowadays, I stick with Sixt as I use them overseas as well.

Long term, over a few weeks to months or longer I would use an independent car rental.

 

Short term, less than a month, I would use the airport rentals and complete online.

 

Good luck.