Disclaimer: Always download from official sources to avoid malware. Here are the verified locations where Marc himself offers free versions.
Marc has a modified version of the famous Artemus brush. It has a slight grain and a dual-round tip. marc brunet advanced brushes free top
If you cannot afford the official set ($15–$25 USD), you can achieve 90% of the same results using these free brush packs that follow the same advanced principles as Marc’s. Disclaimer: Always download from official sources to avoid