Mmm . . . I'm looking for something a little more beguiling, odd, unexpected. As I said, the old mayo jar is acceptable, but dull.
Let's really break the idea of the potion bottle. All it has to be is a reasonably nonporous container of a reasonable size. There's lots of things in the world that fit that description.
For example--the following example isn't really suited to my scalding steam breath potion, but it's a sample of a non-potion bottle container, running free with the phrase "I'm a Little Teapot":
"There's a little teapot back here--one of those tiny oriental iron ones. With the matching lacquered tray, and some cups. And a tin with some, uh, I guess it's tea in it. Smells kind of funny, though."
"Bring it out here."
"Hmm. Ok. Hey look, there's ornate writing laquered into the tray."
"What does it say?
"I'm not sure. I think it's Chinese. Something about herbs? and youth, much youth . . . it's a vitality potion? No, I'm sorry, that's "young," not youth. Much young . . . a virility potion? "
"Let me see that. Yup--you must be right. 'Tea With A Thousand Young.' Maybe it's some kind of herbal Viagra."
"Right."
"So, what next?"
"I guess we boil some water."