My 2001 civic has progressed from slipping out of gear to no longer being able to use gear R,2,4,and 5. its got 130k miles on it. i need to fix it soon, but im not sure what to do. I am pretty sure the entire problem is rooted in the transmission but dont know how big of problem it is. Are there any tests that can help diagnose a specific problem?

It seems like most of the time people replace their transmission with a new or rebuilt one. where is the best place to purchase one? internet store? junk yard?

Also, i have seen transmission rebuild kits but have not found any good kits for my transmission, has anyone found or used one before?