Here are a few websites that offer transcripts for famous TV shows. Subslikescript.com seems to have lots of transcripts for many different shows. Down below are a few series that I have been watching myself.
Subslikescript.com has also transcripts for movies.
ER
Frasier
http://www.kacl780.net/frasier/transcripts/
Friends
https://fangj.github.io/friends/
How I met your mother
The big bang theory
Top Gear