Thanks , for all , good dayI was in Malaysia on my norwegian plated bike in 2015 and I passed a small border crossing of Wang Prachan-Wang Kelian from the Satun province of Thailand. There was an insurance office there but unfortunately it was nobody there that day so I couldnt buy insurance. I then tried as I was advised to by the other border staff to ask at banks along the way but to no avail. I went to the office of one of the main insurance companies in Penang but they couldnt offer me any insurance. I then went on to KL and sent several emails to different insurane companies - I think one answered and said sorry they couldnt offer any insurance.
So I then headed back to the main border of Sadao and after a lot of back and forward I could finally get insurance there.
So my best advice is - cross at the bigger border crossings and chances are best there to find insurance.