I've heard Salmon oil is expensive in Thailand, is that correct?

Would it be ok to bring a 500 tablet box of Salmon oil to Thailand & a box of 1kg powder of "Wagner MSM Joint Formula with MSM Glucosamine & Chondroitin"

Probably a good idea to bring it but there are glucosamine-chondroitin powders and omega-3 capsules available for a reasonable price. Try the pharmacy on the 4th floor of MBK'S Tokyu Dept. Store. If you want to buy a supply in Bangkok. Best to bring your own for a short stay though.

I'd definitely suggest bringing your own supply for a short-term stay. I've found the locally available brands just don't work as well as what I'm used to from the U.S. and are more expensive. You shouldn't have a problem bringing in a reasonable supply for personal use in your luggage for a holiday trip. Be sure to keep it in the original containers in your checked luggage.

Edit: Oops -- Hubby just reminded me that I decided to switch from U.S. sourced omega product to one I get from a local pharmacy last year because it's made from flax seed and doesn't give us fish burps. I tried it when my U.S. shipment was delayed for a week or so during a long Thai holiday. But I still get my glucosamine-chondroitin from the U.S. because a couple joints really started aching when I had to use a locally sourced product during that same delivery delay.