Loose shifter
 

On my fc rx7 my shifter is really loose and shaky. I'm not sure how to fix it and it's hard finding 3rd and 5th.

it may not be your only problem, but at least a part of your problem is the bushings.

https://www.rx7club.com/2nd-generati...return-897994/

It may just be the bushings. See this link: Mazdatrix FAQ - 86-92 Shifter Bushings

If it is not the bushings, then you are probably going to need to take it to a professional transmission repair shop.

i had this same problem and ordered the shifter bushing kit from mazda trix and now it feels awesome! they have great instructions too

i had this problem in my 300zx, there was so much play in the shifter, when it was in gear you can move the shifter like it was in neutral, anyways i just popped some new bushings in and it fixed my problem , was still some play but there was a huge difference in it after i swapped the bushings out

Could also be the Shifter Springer Part: M501-17-465,
I ordered the bushings and the shifter spring, have to drop the transmission to install that one tho.