If you google:  
free funny scripts

... you'll find kazillions. 

e.g: http://www.icomedytv.com/comedy-scripts.aspx

Most of these sites let you perform them with/without acknowledgment of the writer, so just check the terms for each one, as you read through them. These sites also include free jokes for use by anybody. 

Also note, if you are only animating the script for educational purposes (e.g. while learning how to animate, and with no intention to resell any of it), then you can also use scenes from many famous movies via the database here:

http://www.imsdb.com/

Can't wait to see what you come up with!
:)





Anita, aka "Bleetz" the Toon Wrangler @ Tarampa Studios
SYS: Win 8.1,  64-bit, i7-4770 CPU @3.4GHz,  
32GB RAM, Intel(R)Graphics 4600 & nVIDIA GTX1060 TI 

Excellent link Vidi - thank you very much for posting

there are also a lot of suitable stories to be found in public domain comic books:
www.comicbookplus.com