Can you recommend any good agencies?

Hello

Firstly it depends in which city you are going to search an accomodation, and estate agency deal with long term rental usually 12 months in Thailand.
Just search on internet you will find many general websites advertising everything and not a specific agency website.
Others points there are not like in UK sole agency contract for 8/12 weeks to sell an house here, that means anything you find in one estate agency you will find it in many others estate agencies or broker, advertised everywhere.
It is common in Thailand that the same photos and description are sent to many of them to spread the web like a competition between all of them to find a tenant, that means none of them have visited the premise or taken their personal photos shoot, others concerns I have gone through is many websites are not updated, old advert still online years after or after premise is rented, no maintenance, estate agency take longtime to get owner contact or never get them as I said previously nobody is in the pole position to contact the owner or having the keys to visit the house, they are also many independant people freelance, brokers, etc... sometimes better to drive in a mooban and see the post in front of the gate and call the number...

Best of Luck to you