You can use any plain water softener or pool salt in your pool.
I suspect the salinity of your water is all ready pushing 1000 ppm.
To add 1000 ppm salinity to your pool you need to add 140 lbs of salt.
Funny story, I was at HD today getting bleach (only 13 days old...score!), and a lady was next to me pondering the last two bags of pool salt. They were both ripped open and spilling their contents. She walked away, I'm assuming to ask a sales person if there was more pool salt, apparently there wasn't and she came back to tape those two bags closed.
I asked her "Oh are they out of pool salt?" she said yeah can you believe it? Well yeah, everyone is getting their pools ready! I told her I bet they have water softener salt, and it's probably cheaper.
She looked at me so funny...like HEY STUPID, THIS IS FOR A POOL!
I said "No, really, I read you can use it in in the pool, it's the same salt"
"No, no, this is for a POOL, to generate chlorine, totally different kind of salt"
"Okay, well good luck!"
I doubt those two half full bags of salt were going to get her where she needed *shrug*