This is the premier source for the complete method. You can find the PDF version, which contains the foundational exercises, scales, and the renowned 50 studies/études, freely available for download.
The method acts as a ladder. Rather than throwing the student into the deep end, it builds strength, endurance, and technique systematically.
: The IMSLP Lefèvre Portal provides free, scanned PDFs of the original 1802 edition and various modern arrangements, including the "60 Exercises" and "6 Clarinet Sonatas".