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You can't do this and make a sterile solution at home. You'll have to get it from a vet or from a human pharmacy. I would just go to a human pharmacy and ask for LRS (lactated ringers solution). If you use table salt (sodium chloride) in water, you'll end up with an acidic solution that will sting and not be balanced. You really want to use a balanced solution like LRS or Normosol or Plasmalyte.
When you do get something, you can give 10 mL per lb body weight 1-2 times daily for a good 3-7 days. Most places cell these ~ $10 per liter.
You also want to make sure you're giving pedialyte or Gatorade and some canned food by mouth. The only way for the intestinal cells to heal is to get nutrients down orally. You usually have to syringe feed from home every 4 hours to keep food in. They'll vomit a lot of it, but you have to keep with it.