I've never heard of someone getting a "personal" recommendation. If they are your boss at a non-profit then supposedly they have letterhead. I don't think grandma is an appropriate recommendation to receive. Don't you have a supervisor at BBBS?
If the person writing the letter doesn’t have a letterhead or won’t put one on the letter for you it might not be an a good person to write a letter of recommendation.
I would think that where possible it would be nice to have a letterhead, makes the letter look official. The non-profit should have a letterhead. the grandma not so much, but that letter might be more personable and worth more to the reader on a committee.
Grandma's letter could be poorly received so it is a risk to use it. Think about the stereotypes of elderly who are kind and gentle but easily manipulated. If there is any chance her letter fits that criteria (even if it's not true)...
As @gonnif said, a supervisor is probably much more appropriate. Looking at credentials is the only way of measuring credibility that adcoms have, and is probably why they still require 3 letters from professors even though they can clearly see your grades and MCAT scores.