The HL7 C-CDA specification includes samples and sections.
The HL7 CDA C-CDA companion guide has a HL7 github CDA-ccd-companion also includes some samples.
ONC has a site for C-CDA things including validation and scorecard:
https://site.healthit.gov/sandbox-ccda
On that site is a link to a github repo that holds samples from various sources:
https://github.com/chb/sample_ccdas
Additional samples from multiple EHR products are available here:
https://github.com/jddamore/ccda-samples
Synthia is an open-source application for generating large amounts of sample HL7 C-CDA and FHIR data that statistically mirror attributes of the real population:
https://github.com/synthetichealth/synthea
Pre-generated data sets from Synthia are available here:
Samples are also available in the Transformations folder of this repository.