The transport office in Pranburi is ideal, it's even easier if you have a Thai speaker with you, with the IDL you won't have to sit and watch the film on how to ride or drive !!!
Depends how long you want to stay in Thailand I think the IDL is valid here for quite a while, to be honest nobody looks at the date anyway, but if you want to transfer it to a Thai license it needs to be valid.
Get an IDL from your post office before you come only about £5 it's good for a few months then if your staying for good you can easily transfer that to a 2 year Thai license at the transport office.