It is reasonably difficult to rebuild an engine unless you know what your doing, it is very difficult to rebuild an automatic transmission unless you know what your doing. Not trying to be rude and maybe I have underestimated your skill level but generally speaking someone who would be capable of doing these things usually doesn't have to ask where to get the parts/gaskets/clutches/steels/ect. in order to do the job.
I know I made that mistake years ago. I figured that an automatic couldn't be that hard to rebuild. I did a fine job of tearing it down, had a few problems getting it back together so had to ask a friend who works on them to help. I have since learned a good bit more and still I wouldn't want to be rebuilding a transmission if my runner was in need of it right then.
I would get one out of a junk yard to rebuild, do it up and put it in, if you have done your job right then your good to go. If not, well pop the original back in and count the failure as a learning experience and see if you can see what went wrong. Ill never tell someone that they shouldn't try to learn, just saying that if you aren't experienced than please don't assume you will be able to do it easily. Rebuild one before you need it and give it a try, that gives you the chance to make sure it works without leaving yourself without the runner for a while.
Sorry for the long post.