There is no standard for publishing/archiving source code for an open source project. Many organizations use a public repository (e.g. GitHub), others provide access via a download from their site. It is also common to include the source with the install/binaries (e.g. .zip file) or on any distributed media with a product.
The Open Source Initiative, the stewards of the Open Source Definition, and an internationally recognized authority in open source standards and practices, requires access to un-obfuscated source code. So if the organization distributing the software is claiming that that software is open source, they must provide you the source.
A suggestion for your specific case would be to simply contact the company that sold it to you (as @curiousdannii mentioned).