Take a look at the budget you have for the theatrical production. Find out what is set aside for costumes. Based on the amount of money you have available, tailor your search for patterns.
Make a list of the characters for whom you need costumes and the time period of the clothing you are seeking. This will help you determine the source of your patterns. Modern clothes and some vintage ones are readily available while other time periods are not as available and will need you to make most of the costumes.
Measure all the actors, so you know what is needed and wanted in regards to sizes and patterns. Follow the standard measurement charts and procedures for accurate body measuring. Threads is a wonderful website that explains this process quite clearly.
Go to a familiar theatrical costume website like Costume Manifesto. Search the site for theatrical costumes per the period your characters are portraying.
Search the Internet for the patterns you desire to discover other outlets for patterns, making sure you are getting the best deal. There are specialty sites for different periods, characters and themes. Once you find the patterns you need, make sure you understand the purchasing process and the ability to get a refund if by chance one of the patterns doesn't suit your needs.