The very “busy” (hard to read/navigate) French National Monuments website list only one Metro line and five bus lines that will take you to Sainte-Chapelle; bus lines 21, 27, 38, 85 and 96. Lines 21, 38, 85 and 96 will drop you off right in front of St. Chapelle. Line 27 has a stop at St. Michel (along with **every** other bus and a couple of metro lines). If you take line 27 you get to walk/roll across the Pont St. Michel to the Ile de la Cité. Other than the annoying street traffic it’s a nice short stroll.
Visiting St. Chapelle itself might prove more challenging that simply getting there. In the previous century I was able to practically walk right in. Nowadays I see very long queues. If you get to this destination and are discouraged by the long lines, I have an alternative visit possibility.
According the the Wall Street Journal “A forty-year project to restore Sainte–Chapelle’s stained-glass windows will be completed soon.” I recall my 1982 visit when I took a photo (on film!) of this fun English translation of information about the restoration.