I got a sample of a product called "True Lemon" when I made a purchase at
Amazon.com the other day. It's a dehydrated lemon juice product that comes
in small packets, similar in size to artificial sweetener packets. One
packet is good for flavoring about 12 ounces of liquid, such as tea or
water.
I tried making lemonade with it, and it was surprisingly good. It tasted
fairly lemony, although it did not have that good strong lemon oil fragrance
you get when you squeeze your own, and it didn't color the water, either.
I don't know what it costs, and haven't seen it in the stores yet, but I
think it might be a worthwhile product where real lemon juice might be hard
to keep, such as at work, or while camping.