PTA being sales tax exempt

Does anyone know if a PTA can be sales tax exempt? As a 501 Non profit I know we are exempt from Fed income tax but we spend alot of money on retail items and that sales tax adds up. It REALLY added up when we paid the yearbook company and there was 600.00 in tax on it!

I cannot find anyone who can give me guidance on this. Somethings we can contract through the school and have them pay for it and our PTA reimburses them but some things the school board policy prohibits purchases of: items like food for carnival or fundraisier companies so we are on our own.