Virtually all theaters now sell tickets via the Internet.
A good general website to check to know more about what's playing is:
They generally have a link to where you can order tickets from. In the case of Young Frankenstein they link to Ticketmaster.
The TKTS booth that was mentioned by Mark is good if you're not fussy about what you see and are willing to go with whatever they have tickets for on a given day. If you have your heart set on a specific play, it's better to order the tickets in advance.