I'd advise going to the website of your local public library (or, if you're in college, your school's library). Once there, I'd check their catalog for the following
- the DVD of "Sunday"
- the "Sunday" script
- Sondheim's biography by Meryl Seacrest (sp.?)
(it's a shame that 'Look, I Made a Hat' isn't out yet)
- Deconstructing Harold Hill
- the Broadway Best Plays Yearbook (the title's something like that; "Sunday", obviously was a best play for that year
If it's not in your county's catalog, check to see if your library has an inter-library loan system. This will probably yield greater results.
Check Google to see if there's resources that I've missed. Go to the Sondheim website for additional info- see if you can find back archives of the Sondheim review.
Best of luck! I wrote an essay on "Sunday" for a scholarship I'm a finalist for- it's a gold mine of inspiration!
"There are only two worthwhile things to leave behind when we depart this world of ours: children and art."
-Sunday In The Park With George