Is there a 90 day visa? When I got a tourist visa in 2006, the only one available was 60 days with the possibility, for a fee, of extending it thirty days.

I met some people who were traveling around on their '90-day visa'. When I didn't believe they had one, they got out their passports. I then had to explain that 'validity' refers to the amount of time between the date the visa was issued until they would no longer be allowed to enter the country on this visa and would need to apply for a new one. It did not mean they could stay for 90 days. They also thought they had 90 days to use however they wished, and planned to enter and leave the country several times, even though they had a single-entry visa. Unless a visa specifically says that you can enter multiple times, it is good for one entry only. If you pay for a 60-day visa and leave the country on day 10 for a quick trip over the border, your visa is done.

I don't see any way you can come to Thailand without a job and be assured that you will be able to stay for a year unless you can get someone to sponsor you, which will, for them, be a big hassle. English schools, which should be eager to sponsor people so they will have teachers and can make money, don't do it. So I really doubt that anyone will do it for an internship.

Have you looked at being a volunteer with an NGO?