The NGO or the foundation who has invited you are the only ones able to provide the paperwork for your visa. You would need to ask them. They will know the procedure for you. There is only the volunteer visa for this kind of situation and you cannot be working or earning any money in Thailand when you are on it.
The agent needs to give you the original WP10 cancellation form which they received when they cancelled the work permit. They keep a copy. You need this before you can do anything else unfortunately
Just be careful that attending a course whilst visa exempt is allowed. I stand corrected but as far as I know only attending a short course, sport event or conference is allowed. Long training periods need a visa.