Are there any reasonable substitutions for lemon juice? In the past I cooked myself into a corner when I realized mid-recipe that I didn't have any lemons or lemon juice available. Nor did I have any limes. 

I can't remember the exact recipe, but I believe it was some baked fish dish. What are some possible substitutes for lemon juice in this application? I ended up using a dash of apple cider vinegar. It didn't turn out bad, it just was distinctly not lemon.
When you find cheap lemons, buy a lot.  Squeeze half a lemon in each of the cavities of an ice tray.  Freeze.  Within a day, remove the frozen slivers from the tray to a ziploc bag in the freezer. You now have measured units of fresh lemon juice you may use for cooking and will keep for months.  The frozen lemons are a bit less acid than fresh juice, but full of flavor.  You can do the same with limes.