Nationally, there are the "Directory of Grant Making Trusts", "The Grants Register" (published by Palgrave Macmillan) and "Sponsorship Funding Directory" (published by Plymbridge Distributors Ltd) with updated registers and descriptions of foundations and trusts.
Regionally, you may look for "A Guide to Local Trusts in Greater London" (London), "A Guide to Local Trusts in the Midlands" (Midlands), "A Guide to Local Trusts in the North of England" (North of England), "A Guide to Local Trusts in the South of England" (for the South of England). Those guides should normally be available in your local library.
One general foundation that funds young individuals and their volunteering trip is "The Jack Petchey Foundation" which holds grants for young people between 11-25. A grant is about £300. The Association of Charitable Foundations provides links to different trusts and foundation websites; the Directory of Social Change provides various databases such as "Grants for Individuals" where you probably find suitable trusts.
Besides trusts and grants, there are other rewarding ways to fund your trip. Find individual sponsors: Ask family, friends, relatives, companies and everybody who might be able to support you. Inform about your project and your goals: create posters, contact your local newspaper or radio, use social networks and the newsletter of your school. Write presentation letters about yourself, your project and your goals to local businesses that might be interested in sponsoring a volunteer trip, and offer them reports, presentations, talks, etc. as compensation. Organizing Fundraising events is another useful and rewarding way: organize a marathon, a concert, whatever you have in mind. Spread the word in your local newspaper (your local community might be helpful in terms of organising such events, you just need to ask), get friends to participate, spread your event in your social network to get people to participate. It needs a lot of planning but it may be very effective in the end! Your school or university can be a very effective source, too: ask about possible financial assistance from your school or university; they might have scholarship opportunities or grants for a stay abroad or specific projects or they may have further ideas and contacts. Most universities already provide information about grants and funds for a stay abroad on their website.
USA and Canada
Your school / college / university can be very effective resources, too: use school newsletters to promote your project, aks your guidance counselor / students supervisor about possible financial assistance from your school / college / university (they might have scholarship opportunities or grants for a stay abroad). Many academic departments may have some budget to fund your project, in case it can be integrated into an academic course for example. Colleges may have funds for alumni service projects and may require a report about your stay for the alumni journal. You may also set up educational talks about the country, about your volunteer project, about your experiences, etc. in exchange for some small contribution. Be creative and most of all: ask for advice and opportunities! Universities mostly provide useful information about funding programes for their students already on their website.
IEFA is a useful resource to find any kind of grant information for students who wish to go abroad. Lots of scholarship opportunities as well as grant listings are given. Foundation grants to individuals online is a bit easier to use: you will not only have a list of foundations and scholarship opportunities but can search with lots of individual details matching your needs. The website further provides regularly updates on new and existing programes, search tips and strategies for finding the right grant and lots more. Due to its extended service, you will need to register and pay a little fee to use the whole range of service.
For canadian volunteers, there is a useful database of Yahoo! Canada with some foundations and trusts listed, which refer to grants, funding and scholarship.
For funding / scholarships in Germany, Austria and Switzerland please refer to the German language version of this article.