Before you dig in too far I have a bit of advice. I have an automatic so I can't comment on the 7th gen transmission specifically. I have seen this on several Hondas including one of my own and I believe the most common reason to have the transmission slip out of gear is lack of proper service. IE either not changing the transmission fluid or being low. Honda MTF is preferred, GM Synchromesh works great as well (I use it in my 90 Integra b/c the gm dealer is a lot closer to my house than Honda. When you change it be sure to clean the drain plug well, its magnetic and the only real protection against metal fragments and shavings since its basically an oil bath without a filter. If you notice big chunks then you know you have big problems.
I had a friend with a 90 Ef hatch who was convinced his transmission was toast, turns out the fluid looked like it had never been changed. The difference was night and day.
Of course if there is bearing damage then its toast, but I believe its worth the $20 or so to try new fluid before even considering replacing the transmission.
car-part.com is a national network of salvage yards that post their inventory online. I bet you wouldn't need to drive incredibly far to get one. If you want to drive down to Columbus, Ohio I can recommend a couple decent places.