I realize I won't be able to get it for free, but is there somewhere I can purchase this book in e-book format?

You are not the first person to ask this question:
See

http://www.mobileread.com/forums/showthread.php?t=9638

I've done a Google search in the obvious places and don't believe it is available. But I see that you have a Kindle. Why not email a request to Amazon?

There is no official version available as an ebook. You can get the Cliff's Notes for it as an ebook though. ;)

AFAIK, Penguin is the publisher for it & they're very hit or miss as far as ebooks go.

Actually, the Centennial Edition is beautiful, elegant, and affordable: link to Amazon (http://www.amazon.com/Grapes-Wrath-Centennial-John-Steinbeck/dp/0142000663/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1201755590&sr=8-1)

I just finished it a couple days ago and it is still resonating in my head. I didn't mind reading it in paper at all. What a wonderful novel. Just buy it, you won't regret it. :)