For locally made, there's Home Motor near Mae Rim 053297771 / 0871894608 - plenty of posters here have used them. I got a full system for a KLX150 for 3500b which I'd say is pretty good. Or
there's Mr Kwan near the Super highway 0871926091 who also does exhausts. I got a skid plate from him but he was busy making (I think) Yoshy copies for the CRF250. Can't vouch for exhausts but you can see some of his other CRF250 bits here:
honda-f61/2012-honda-crf250l-cbr250r-goes-dual-sport-t1044-160.html. Thinks you'll need a Thai speaker to help - if you're not.
Edit - if you want quiet though, you're maybe going to have to figure some kind of insert/muffler. My exhaust is loud. best find someone in CM with one from them to check how loud they are...Or explain you want something not too loud and see what they say.
Edit 2 - there are pics of the Yoshy copy here, by the looks of it:
honda-f61/2012-honda-crf250l-cbr250r-goes-dual-sport-t1044-140.html