How do i start taking legal action against a pet shop that sold me a sick pup and a the vet that neglected him

1 week after i got titan (puppy) he became really ill and so i took him to the emergency clinic (230$), later that day he was diagnosed with parvo. i immediately alerted the pet shop, and the owner admitted to knowing that there was a high risk that titan had parvo and yet he didnt tell us. he agreed to pay the bill on the condition that we took titan to his vet. titan was there for 3 days, the 1st 2 days i called 3-4 times a day to check on him on the 3rd day i went to see him, i was horrified to see that he was placed in an unquarintined room (laundry) with mops and boxes surrounding him. he was in a tiny cage with one layer of newspaper wet in his own vommit, and no IV attached. i took him out of there immediately to another vet (1500$ for treatment). due to the dodgy vet titan became hypoglycemic and had a seizure which caused temp. blindness and brain damage (vision still blury and walks wobly), now he has megaesophagus due to the parvo and no insurance company will ever take him.