Substitute for fresh lemons - I need to make 70 liters of homemade lemonade
You can very likely buy frozen lemon juice or juice concentrate, even if you can't buy fresh lemons.  (This is different from frozen lemonade concentrate.) 

This is what we do often when we're making large batch lemonade; while it's not quite as good as fresh lemon juice, it's usually close enough that it's not noticeable.

For example:



It's kept in the freezer section. That jar is roughly seven lemons' worth of juice; each one makes something like 2-3 liters of lemonade, depending on your strength requirements.  I usually put a few sliced fresh lemons in with the pitcher after that.