I want to buy a rugged cellphone in thailand but where do they sell them? Something like Blackview or Ulefone....

 

And what if i buy one online from overseas, is there much importtax on them?

Banggood.com has them and in the past I found them to be reliable.  I was never charged duty on a phone I purchased from Banggood.  I made sure to use their postal option, never FedEx or DHL. Delivery usually within 10 days. I never bought a phone that cost more than US $200 so that may have been part of the reason for no duty.   I was charged duty on a couple of tablets I purchased.

Shopee have some CAT phones on there, that might be worth a look. 

I suggest any phone will do...just carry it around in a metal box so you cant trash it since your so carless....