ncprasad, I get reviewers from various sources. I have 2-3 friends who are good writers to review as non-MBA folks perspective.
I have 2-3 Haas friends who are willing to review my essays (more if I asked and if I need to ask, maybe for a final draft review). I have so many I guess because of luck and my networking using my Cal alumni status. Cal students are also extremely friendly, so if you get to know some of them well, eventually they should be willing to check your essays out.
For Stanford, I only have 1 alumni I talked with that volunteered to read my essays when I'm done, so I'm not going to say no!

I might ask one more student if I connect with him once I meet him.
For UCLA, no reviewers yet. But I may make some friends soon.
Kellogg I have one so far who I haven't asked, but he probably would be willing to read a final draft for minor comments if I asked..
I guess in the end you decide by how well you connect with the person and how friendly they are. All 4 schools I'm applying to value team work, and two of them are my alma maters, so I guess I'm lucky in that way.