if you've done any volunteering with someone who has a current connection to the work force, idealy in a supervisory or managment capacity you could ask them to write YOU a letter to keep on hand.
we had to do this where i worked because the policy was we could'nt write letters of reccommendation or let people list us as references-this is becoming common with public agencies and organizations so always check with anyone associated with a school or a charity to see if they can be listed by you.
what i would do is have someone tell me that they were looking for work in a specific field and then ask me to put in writing what what my opinion was. i'de then write a letter to them that would say something like this:
dear sally,
you've shared with me your decision to seek employment as a ____________ and asked my opinion.
i want to share with you that when we both served on the __________ fund raising committee i was very impressed by the professional manner in which you conducted yourself.
i observed your excellent personal communication skills as you worked with other committee members and volunteers. your organization, preparation and presentation of the reports that kept committee members and volunteers aware were outstanding and a key factor in our group's success. you displayed knowledge of business principals by your attention to detail and committment to staying on schedual. i always appreciated your perfect attendance at every meeting and event. as a person who professionaly supervises staff that are responsible for many of the same responsibilities you undertook i want to share that had one of my staff members performed in the manner you did i would be extreemly appreciative to have them as a member of my team.
something like this-it commends you, spells out what you have done at no pay but relates it to a paying position.
good luck!!!