Aqua Hydrate brand water bottles, not as light as the normal pet bottles, but they are very good quality, 17oz weighs 30 gm, 33.8 oz weighs 41 gm, but the caps feature an inner lip for a very good seal. These are hard to find, I only know of one healthfood store that carries them in my area, The Food Mill (oakland).
The bottom is round and flat, like a regular water bottle. They have a variety of sizes, I got them all because they are such good bottles.
the 33.8 oz is as narrow as the 17oz, which means you can carry it in a small pocket, it's tall. They also come with good quality sport top drinking tops too. The 20oz is also very narrow.
Slightly worse than these are the trader joes 'electrolyte enhanced water' bottles, heavier too, 51 gram, but also flat bottomed, the cap is not as good. Wholefoods sells the same thing with their generic brand name on it.
I'm glad to see I'm not the only one with boxes of bottles and repurposed containers.
There's a tiny bottle I found once, it's breath mint candy, it's basically I think a 3ml squirt bottle, like lawson sold, soak for a long time to get rid of the smell. Don't remember the brand, it's ice something, but it's probably cheaper to just buy the new bottles.