The only "jar" that you might be able to keep a small fish in is a large battery jar. They are all glass jars made to assemble large industrial batteries in and can sometimes be several gallons in capacity. One of those might suffice for small fish that get frequent water changes, much like a small aquarium would need. These days, even large industrial batteries are being made with clear plastic cases and I doubt that an old fashioned battery jar could be found newly manufactured in the last several years.