Aquafina bottles work. But I always worry about puncturing a hole in them. If you are comfortable that you wont stick a hole in it, it is the best option and they a free after you drink the water :)
Platypus discontinued their .5L Lil Nipper bottle. At .7 oz it was a good product. I would suspect that a regular .5L Platypus water bottle should work. Short of that you could get a red Platypus wine bottle. they hold about 800ml and weigh .9 ounce. You won't confuse it for a water bottle either. I have one that I occasionly use when I feel like taking a little wine with me on a short trip and know I will be enjoying a great sunset. I use Platypus bottles for my water, but am still a little aprehensive about poking a hole in them. I know they are supposed to be pretty sturdy, but I take extra care with them anyway.
Lastly you can make a gasket for you present bottle. Get a Zip Lock brand sandwich bag (cut off the zip top). Now cut a small square and cover the top and threads of your bottle, and then screw on the top. Take the rest of the bag with you. It doesn't even register on my postal scale. If and when you damage the gasket after a few uses, just cut another square from the remaining zip lock bag. You can probably make at least a dozen gaskets from one bag. I did this this a couple of times with no problems. I was worried that the alcohol would melt the plastic, but I didn't have any problem with that.
I just bought some of the blue 8oz flip bottles from antigravitygear(weight = 1.2 ounces ea) and there is no confusing them for something else. They didn't leak when I tested them at home, but I pack fuel bottles straight up anyway. They are not super light, but the last thing I need is for my fuel to leak out.