The normal stop bath is indeed acetic acid.(posted 9206 days ago)The pure stuff is pretty corrosive: dilute it to about 1+200. The stuff you buy in photo shops is already diluted about 1+3. Or take some vinegar, and dilute it something like, I dunno, 1+60.
If you really want to create your own chemistry, I suggest you read some books about it first. There are many hazards for the unwary.