You can’t buy tickets separately to the Sistine Chapel.
Tickets to the Sistine Chapel and the Vatican museums go together and it is always better to book them online in 2-3 weeks before your visit to Rome.
The entry price is 17 euro + 4-euro commission if you buy online. However, it is worth to overpay this commission because you will skip several hours of standing in a queue. Moreover, it is much better to visit the Sistine Chapel and the museums with a licensed tour guide. If you didn’t have a chance to buy a ticket in advance, you can also find tour guides directly on St. Peter’s square and go inside the museums with a group of tourists.