Exam English is a free resource site. While they don't always offer full 100-question tests back-to-back, they offer "Part-by-Part" practice. Do all parts for each section back-to-back to simulate a test.
Many national libraries have digital lending programs. often holds older editions of Cambridge preparation books. Since the KET was renamed to A2 Key a few years ago, the older "KET" editions are often out-of-print and available for legal borrowing. You can "borrow" the digital scan for one hour at a time to complete a test. succeed in cambridge ket 10 practice tests free download