Who would have my immunization records if I lost mine?

Schools and Doctor. The record may be available from your school system. Yor doctor should have them. And some states have a state immunization registry. And ask your parents.
Depends. In my state a doc must keep the record for 7 years after your last visit and or your 21st birthday (if you have not been in for years). Schools, camps, employment applications, may also have records.Some parents keep them with your school mementos. .