I have a bad transmission leak in my 1995 plymouth neon. My mechanic suggested some kind of stop leak?

its very rare when this ever works to stop a leak,there no substituting a new seal over a sealant,i haven't ever used it and wont ,because i have heard some real horror stories on this stuff,i doubt that it will help you much,you,ll more than likely have to get the seal replaced in it,good luck on it.