bk wrote:

Where do I get this salt?

Try the hardware stores, maybe the bulk foods outlets, places that
sell to commercial fishermen. 50# bags of salt should run less than
$10/bag.

Coarse salt is used on roads, sometimes. It's cheap, but messes up
cars and roadside vegetation. Fine is generally used in food processing.

The coarse salt is slower to dissolve, but is "better behaved" in that
the grains are less likely to float out into areas where you don't
want grains of salt. I'd put it in a mesh bag and hang it in front of
the water flow into the pool. The fine salt should be disolved in a
bucket, a bit at a time, allowed to settle, then the brine poured into
the pond. That's a lot of fussing around.

Dumping the salt straight in could result in a fresh water layer on
the surface and a potentually lethal concentration of brine near the
bottem. Even broadcasting fine salt over the surface seems a bit
risky, especially in cold water. Coarse salt would be worse.